Semiconductor device, method for manufacturing the same, or display device including the same

ABSTRACT

To suppress a change in electrical characteristics in a transistor including an oxide semiconductor film. The transistor includes a first gate electrode, a first insulating film, an oxide semiconductor film, a source electrode, a drain electrode, a second insulating film, a second gate electrode, and a third insulating film. The oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side, and a second oxide semiconductor film over the first oxide semiconductor film. The first oxide semiconductor film and the second oxide semiconductor film include In, M, and Zn (M is Al, Ga, Y, or Sn). In a region of the second oxide semiconductor film, the number of atoms of In is smaller than that in the first oxide semiconductor film. The second gate electrode includes at least one metal element included in the oxide semiconductor film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductor device including an oxide semiconductor film and a display device including the semiconductor device. Another embodiment of the present invention relates to a manufacturing method of the semiconductor device including an oxide semiconductor film.

Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a storage device, a driving method thereof, or a manufacturing method thereof.

In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a memory device are each an embodiment of a semiconductor device. An imaging device, a display device, a liquid crystal display device, a light-emitting device, an electro-optical device, a power generation device (including a thin film solar cell, an organic thin film solar cell, and the like), and an electronic device may each include a semiconductor device.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor. As another material, an oxide semiconductor has been attracting attention.

A reliable semiconductor device in which stable electrical characteristics are applied to a transistor using an oxide semiconductor is disclosed (e.g., see Patent Document 1). In the semiconductor device, oxide semiconductor films with different compositions are stacked so that an oxide semiconductor film containing much In is provided on the channel side and an oxide semiconductor film containing many stabilizers such as Ga is provided on the back-channel side.

REFERENCE Patent Document [Patent Document 1] Japanese Published Patent Application No. 2013-175715 SUMMARY OF THE INVENTION

The oxide semiconductor film containing many In might have a small energy band gap (Eg) (for example, smaller than 3.0 eV). In this case, an oxide semiconductor film with larger Eg (for example, 3.0 eV or more and 3.5 eV or less) is more influenced by light than an oxide semiconductor film with smaller Eg. For example, as a result of a bias-temperature stress test with application of a negative bias voltage and light irradiation (negative GBT test with light), reliability of a transistor including the oxide semiconductor film with smaller Eg is sometimes reduced.

The negative GBT stress test with light irradiation is one kind of accelerated test and can evaluate, in a short time, change in characteristics of transistors, which is caused by long-term use. In particular, the amount of shift in threshold voltage (ΔV_(th)) of the transistor between before and after a negative GBT stress test with light irradiation is an important indicator for examining reliability. The smaller the shift in the threshold voltage (ΔV_(th)) between before and after a negative GBT stress test with light irradiation is, the higher the reliability of the transistor is.

In view of the foregoing problems, an object of one embodiment of the present invention is to suppress a change in electrical characteristics and to improve reliability in a transistor including an oxide semiconductor film. In particular, an object is to suppress a change in electrical characteristics and to improve reliability in a transistor including an oxide semiconductor film containing much In. An object of one embodiment of the present invention is to provide a semiconductor device with low power consumption. An object of one embodiment of the present invention is to provide a novel semiconductor device. An object of one embodiment of the present invention is to provide a method for manufacturing a novel semiconductor device. An object of one embodiment of the present invention is to provide a novel display device.

Note that the description of the above object does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Objects other than the above objects will be apparent from and can be derived from the description of the specification and the like.

One embodiment of the present invention is a semiconductor device including a transistor including a first gate electrode, a first insulating film over the first gate electrode, an oxide semiconductor film over the first insulating film, a source electrode electrically connected to the oxide semiconductor film, a drain electrode electrically connected to the oxide semiconductor film, a second insulating film over the oxide semiconductor film, a second gate electrode over the second insulating film, and a third insulating film over the second gate electrode. The oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side, and a second oxide semiconductor film over the first oxide semiconductor film. The first oxide semiconductor film and the second oxide semiconductor film include In, M, and Zn (M is Al, Ga, Y, or Sn). In a region of the second oxide semiconductor film, the number of atoms of the In is smaller than that in the first oxide semiconductor film. The second gate electrode includes at least one metal element included in the oxide semiconductor film.

Another embodiment of the present invention is a semiconductor device including a transistor including a first gate electrode, a first insulating film over the first gate electrode, an oxide semiconductor film over the first insulating film, a source electrode electrically connected to the oxide semiconductor film, a drain electrode electrically connected to the oxide semiconductor film, a second insulating film over the oxide semiconductor film, a second gate electrode over the second insulating film, and a third insulating film over the second gate electrode. The second gate electrode is electrically connected to the first gate electrode through an opening portion in the first insulating film and the second insulating film. The oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side, and a second oxide semiconductor film over the first oxide semiconductor film. The first oxide semiconductor film and the second oxide semiconductor film include In, M, and Zn (M is Al, Ga, Y, or Sn). In a region of the second oxide semiconductor film, the number of atoms of the In is smaller than that in the first oxide semiconductor film. The second gate electrode includes at least one metal element included in the oxide semiconductor film.

In the above embodiment, it is preferable that the first oxide semiconductor film include a region satisfying In>M (M is Al, Ga, Y, or Sn) and that the second oxide semiconductor film include a region satisfying In≦M (M is Al, Ga, Y, or Sn).

In the above embodiment, it is preferable that the second gate electrode include a third oxide semiconductor film and a fourth oxide semiconductor film over the third oxide semiconductor film, that the third oxide semiconductor film include a region satisfying In≦M (M is Al, Ga, Y, or Sn), and that the fourth oxide semiconductor film include a region satisfying In≧M (M is Al, Ga, Y, or Sn).

In the above embodiment, it is preferable that the oxide semiconductor film include a crystal part having c-axis alignment.

In the above embodiment, it is preferable that the third insulating film include one or both of hydrogen and nitrogen.

Another embodiment of the present invention is a display device including the semiconductor device according to any one of the above embodiments, and a display element. Another embodiment of the present invention is a display module including the display device and a touch sensor. Another embodiment of the present invention is an electronic device including the semiconductor device according to any one of the above embodiments, the display device, or the display module; and an operation key or a battery.

Another embodiment of the present invention is a method for manufacturing a semiconductor device, including the steps of forming a first gate electrode, forming a first insulating film over the first gate electrode, forming a first oxide semiconductor film over the first insulating film, forming a second oxide semiconductor film over the first oxide semiconductor film, forming a source electrode and a drain electrode over the second oxide semiconductor film, forming a second insulating film over the second oxide semiconductor film, the source electrode, and the drain electrode, forming a third oxide semiconductor film functioning as a second gate electrode, over the second insulating film, and forming a third insulating film including hydrogen, over the third oxide semiconductor film. In the step of forming the first oxide semiconductor film, the first oxide semiconductor film is formed in an atmosphere including a first oxygen gas, and the first oxygen gas is added to the first insulating film. In the step of forming the second oxide semiconductor film, the second oxide semiconductor film is formed in an atmosphere including a second oxygen gas. In the step of forming the third oxide semiconductor film, the third oxide semiconductor film is formed in an atmosphere including a third oxygen gas, and the third oxygen gas is added to the second insulating film. In the step of forming the third insulating film or after the step of forming the third insulating film, the hydrogen is added from the third insulating film to the third oxide semiconductor film.

In the above embodiment, it is preferable that the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film be each formed by a sputtering method.

Another embodiment of the present invention is a method for manufacturing a semiconductor device, including the steps of forming a first gate electrode, forming a first insulating film over the first gate electrode, forming a first oxide semiconductor film over the first insulating film, forming a second oxide semiconductor film over the first oxide semiconductor film, forming a source electrode and a drain electrode over the second oxide semiconductor film, forming a second insulating film over the second oxide semiconductor film, the source electrode, and the drain electrode, forming a third oxide semiconductor film functioning as a second gate electrode, over the second insulating film, forming a fourth oxide semiconductor film functioning as a second gate electrode, over the third oxide semiconductor film, and forming a third insulating film including hydrogen, over the fourth oxide semiconductor film. In the step of forming the first oxide semiconductor film, the first oxide semiconductor film is formed in an atmosphere including a first oxygen gas, and the first oxygen gas is added to the first insulating film. In the step of forming the second oxide semiconductor film, the second oxide semiconductor film is formed in an atmosphere including a second oxygen gas. In the step of forming the third oxide semiconductor film, the third oxide semiconductor film is formed in an atmosphere including a third oxygen gas, and the third oxygen gas is added to the second insulating film. In the step of forming the fourth oxide semiconductor film, the fourth oxide semiconductor film is formed in an atmosphere including a fourth oxygen gas. In the step of forming the third insulating film or after the step of forming the third insulating film, the hydrogen is added from the third insulating film to the fourth oxide semiconductor film.

In the above embodiment, it is preferable that the first oxide semiconductor film, the second oxide semiconductor film, the third oxide semiconductor film, and the fourth oxide semiconductor film be each formed by a sputtering method.

In the above embodiment, it is preferable that the flow rate of the third oxygen gas be higher than the flow rate of the fourth oxygen gas.

With one embodiment of the present invention, a change in electrical characteristics can be suppressed and reliability can be improved in a transistor including an oxide semiconductor film. In particular, a change in electrical characteristics can be suppressed and reliability can be improved in a transistor including an oxide semiconductor film containing much In. Alternatively, with one embodiment of the present invention, a highly reliable semiconductor device can be provided. Alternatively, with one embodiment of the present invention, a novel semiconductor device can be provided. Alternatively, with one embodiment of the present invention, a method for manufacturing a novel semiconductor device can be provided. Alternatively, with one embodiment of the present invention, a novel display device can be provided.

Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 2A to 2C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 3A to 3C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 4A to 4C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 5A to 5D are cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 6A and 6B illustrate a band structure.

FIGS. 7A to 7D are cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 8A to 8F are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 9A to 9F are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 10A to 10F are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 11A to 11F are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 12A and 12B are model diagrams illustrating oxygen moving in an oxide semiconductor film.

FIG. 13 shows TDS measurement results.

FIGS. 14A and 14B show TDS measurement results.

FIGS. 15A and 15B show TDS measurement results.

FIG. 16 shows measurement temperature dependence of an oxide conductive film.

FIGS. 17A to 17D are Cs-corrected high-resolution TEM images of a cross section of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS.

FIGS. 18A to 18D are Cs-corrected high-resolution TEM images of a plane of a CAAC-OS.

FIGS. 19A to 19C show structural analysis of a CAAC-OS and a single crystal oxide semiconductor by XRD.

FIGS. 20A and 20B show electron diffraction patterns of a CAAC-OS.

FIG. 21 shows a change of crystal parts of an In—Ga—Zn oxide owing to electron irradiation.

FIG. 22 shows a deposition method of a CAAC-OS.

FIGS. 23A to 23C illustrate a crystal of InMZnO₄.

FIGS. 24A to 24F illustrate a deposition method of a CAAC-OS.

FIGS. 25A to 25G are diagrams showing positions where a particle is attached to a pellet.

FIGS. 26A to 26G are diagrams showing positions where a particle is attached to a pellet.

FIGS. 27A and 27B are a top view showing one embodiment of a display device and a circuit diagram showing one embodiment of a pixel.

FIG. 28 is a top view showing one embodiment of a pixel.

FIG. 29 is a cross-sectional view showing one embodiment of a pixel.

FIG. 30 is a cross-sectional view showing one embodiment of a pixel.

FIG. 31 is a top view showing one embodiment of a pixel.

FIG. 32 is a cross-sectional view showing one embodiment of a pixel.

FIG. 33 is a cross-sectional view showing one embodiment of a pixel.

FIG. 34 is a top view showing one embodiment of a pixel.

FIG. 35 is a cross-sectional view showing one embodiment of a pixel.

FIG. 36 is a cross-sectional view showing one embodiment of a pixel.

FIG. 37 is a top view showing one embodiment of a pixel.

FIG. 38 is a cross-sectional view showing one embodiment of a pixel.

FIG. 39 is a cross-sectional view showing one embodiment of a pixel.

FIG. 40 is a cross-sectional view showing one embodiment of a pixel.

FIG. 41 is a top view showing one embodiment of a pixel.

FIG. 42 is a cross-sectional view showing one embodiment of a pixel.

FIG. 43 is a top view showing one embodiment of a pixel.

FIG. 44 is a cross-sectional view showing one embodiment of a pixel.

FIG. 45 is a circuit diagram showing one embodiment of a pixel.

FIGS. 46A and 46B are a block diagram and a circuit diagram illustrating a display device.

FIGS. 47A and 47B are a top view and a cross-sectional view illustrating a display device.

FIGS. 48A and 48B are perspective views showing an example of a touch panel.

FIGS. 49A and 49B are cross-sectional views showing examples of a display device.

FIG. 50 is a cross-sectional view illustrating an example of a touch sensor.

FIGS. 51A and 51B are cross-sectional views showing examples of a touch panel.

FIGS. 52A and 52B are a block diagram and a timing chart of a touch sensor.

FIG. 53 is a circuit diagram of a touch sensor.

FIGS. 54A and 54B illustrate display of an image on a display device of one embodiment of the present invention.

FIGS. 55A and 55B illustrate display of an image on a display device of one embodiment of the present invention.

FIGS. 56A to 56E illustrate an example of a method for displaying images on a display device according to Embodiment.

FIGS. 57A to 57E illustrate an example of a method for displaying images on a display device according to Embodiment.

FIG. 58 is a view illustrating a display module.

FIGS. 59A to 59G illustrate electronic devices.

FIGS. 60A and 60B are perspective views showing a display device.

FIG. 61 illustrates a structure of a deposition apparatus.

FIGS. 62A and 62B each illustrate I_(d)-V_(g) characteristics of a transistor.

FIG. 63 illustrates I_(d)-V_(g) characteristics of a transistor.

FIGS. 64A and 64B illustrate GBT test results of a transistor and repeated GBT test results of a transistor.

FIG. 65 is a cross-sectional STEM image of a transistor.

FIGS. 66A and 66B illustrate a structure used for calculation and calculation results of current density distribution.

FIGS. 67A and 67B illustrate a structure used for calculation and I_(d)-V_(g) characteristics of a transistor.

FIGS. 68A and 68B each illustrate calculation results of I_(d)-V_(g) characteristics of a transistor.

FIGS. 69A and 69B each illustrate probability distribution of the threshold voltage of a transistor.

FIGS. 70A and 70B each illustrate probability distribution of the threshold voltage of a transistor.

FIGS. 71A and 71B each illustrate calculation results of the variation in I_(d)-V_(g) characteristics in a subthreshold region of a transistor.

FIGS. 72A and 72B each illustrate a structure used for calculation.

FIGS. 73A and 73B each illustrate potential distribution in thickness direction when NBTS is applied.

FIGS. 74A to 74C are a top view and cross-sectional views of a transistor.

FIGS. 75A to 75C are a top view and cross-sectional views of a transistor.

FIGS. 76A and 76B each illustrate I_(d)-V_(g) characteristics of a transistor.

FIGS. 77A and 77B each illustrate I_(d)-V_(g) characteristics of a transistor.

FIGS. 78A and 78B each illustrate I_(d)-V_(g) characteristics of a transistor.

FIGS. 79A and 79B each illustrate I_(d)-V_(g) characteristics of a transistor.

FIGS. 80A and 80B each illustrate I_(d)-V_(d) characteristics of a transistor.

FIGS. 81A to 81C each illustrate I_(d)-V_(d) characteristics of a transistor.

FIGS. 82A to 82C each illustrate I_(d)-V_(d) characteristics of a transistor.

FIG. 83 illustrates I_(d)-V_(g) characteristics of a transistor.

FIGS. 84A and 84B illustrate maximum field-effect mobility of transistors and the threshold voltages of transistors.

FIGS. 85A and 85B each illustrate distribution of a potential applied to a transistor.

FIG. 86 is a conceptual diagram illustrating a potential applied to a transistor.

FIG. 87 is a conceptual diagram illustrating a potential applied to a transistor.

FIGS. 88A and 88B are conceptual diagrams each illustrating a potential applied to a transistor.

FIG. 89 illustrates probability distribution of the threshold voltages of transistors.

FIG. 90 illustrates probability distribution of the threshold voltages of transistors.

FIG. 91 illustrates calculation results of I_(d)-V_(g) characteristics of transistors.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be explained below with reference to the drawings. However, the present invention is not limited to description to be given below, and it is to be easily understood that modes and details thereof can be variously modified without departing from the purpose and the scope of the present invention. Accordingly, the present invention should not be interpreted as being limited to the content of the embodiments below.

Note that the position, the size, the range, or the like of each structure illustrated in drawings and the like is not accurately represented in some cases for simplification. Therefore, the disclosed invention is not necessarily limited to the position, the size, the range, or the like disclosed in the drawings and the like.

Note that the ordinal numbers such as “first”, “second”, and the like in this specification and the like are used for convenience and do not denote the order of steps or the stacking order of layers. Therefore, for example, description can be made even when “first” is replaced with “second” or “third”, as appropriate. In addition, the ordinal numbers in this specification and the like are not necessarily the same as those which specify one embodiment of the present invention.

Note that in this specification, terms for describing arrangement, such as “over” “above”, “under”, and “below”, are used for convenience in describing a positional relation between components with reference to drawings. Further, the positional relation between components is changed as appropriate in accordance with a direction in which each component is described. Thus, there is no limitation on terms used in this specification, and description can be made appropriately depending on the situation.

In describing structures of the invention with reference to the drawings in this specification and the like, common reference numerals are used for the same portions in different drawings.

The “semiconductor device” in this specification and the like means all devices which can operate by utilizing semiconductor characteristics. A semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a memory device are each an embodiment of a semiconductor device. An imaging device, a display device, a liquid crystal display device, a light-emitting device, an electro-optical device, a power generation device (including a thin film solar cell, an organic thin film solar cell, and the like), and an electronic device may each include a semiconductor device.

In this specification and the like, a “semiconductor” includes characteristics of an “insulator” in some cases when the conductivity is sufficiently low, for example. Further, a “semiconductor” and an “insulator” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “insulator” is not clear. Accordingly, a “semiconductor” in this specification and the like can be called an “insulator” in some cases. Similarly, an “insulator” in this specification and the like can be called a “semiconductor” in some cases. An “insulator” in this specification and the like can be called a “semi-insulator” in some cases.

In this specification and the like, a “semiconductor” includes characteristics of a “conductor” in some cases when the conductivity is sufficiently high, for example. Further, a “semiconductor” and a “conductor” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “conductor” is not clear. Accordingly, a “semiconductor” in this specification can be called a “conductor” in some cases. Similarly, a “conductor” in this specification and the like can be called a “semiconductor” in some cases.

In this specification and the like, a transistor is an element having at least three terminals of a gate, a drain, and a source. In addition, the transistor has a channel region between a drain (a drain terminal, a drain region, or a drain electrode) and a source (a source terminal, a source region, or a source electrode), and current can flow through the drain region, the channel region, and the source region. Note that in this specification and the like, a channel region refers to a region through which current mainly flows.

Further, functions of a source and a drain might be switched when transistors having different polarities are employed or a direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be switched in this specification and the like.

Note that the channel length refers to, for example, a distance between a source (source region or source electrode) and a drain (drain region or drain electrode) in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other or a region where a channel is formed in a top view of the transistor. In one transistor, channel lengths in all regions are not necessarily the same. In other words, the channel length of one transistor is not limited to one value in some cases. Therefore, in this specification and the like, the channel length is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.

The channel width refers to, for example, the length of a portion where a source and a drain face each other in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other, or a region where a channel is formed. In one transistor, channel widths in all regions are not necessarily the same. In other words, the channel width of one transistor is not limited to one value in some cases. Therefore, in this specification and the like, the channel width is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.

Note that in this specification and the like, the expression “electrically connected” includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Examples of an “object having any electric function” are a switching element such as a transistor, a resistor, an inductor, a capacitor, and elements with a variety of functions as well as an electrode and a wiring.

A voltage usually refers to a potential difference between a given potential and a reference potential (e.g., a source potential or a ground potential (GND)). Therefore, a voltage can also be referred to as potential.

Note that in this specification and the like, a silicon oxynitride film refers to a film in which the proportion of oxygen is higher than that of nitrogen. The silicon oxynitride film preferably contains oxygen, nitrogen, silicon, and hydrogen in the ranges of 55 atomic % to 65 atomic %, 1 atomic % to 20 atomic %, 25 atomic % to 35 atomic %, and 0.1 atomic % to 10 atomic %, respectively. Furthermore, a silicon nitride oxide film refers to a film in which the proportion of nitrogen is higher than that of oxygen. The silicon nitride oxide film preferably contains nitrogen, oxygen, silicon, and hydrogen in the ranges of 55 atomic % to 65 atomic %, 1 atomic % to 20 atomic %, 25 atomic % to 35 atomic %, and 0.1 atomic % to 10 atomic %, respectively.

In this specification and the like, the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases.

In this specification, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, the term “substantially parallel” indicates that the angle formed between two straight lines is greater than or equal to −30° and less than or equal to 30°. In addition, the term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly also includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. The term “substantially perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 60° and less than or equal to 120°.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of the present invention and a method of manufacturing the semiconductor device are described with reference to FIGS. 1A to 1C, FIGS. 2A to 2C, FIGS. 3A to 3C, FIGS. 4A to 4C, FIGS. 5A to 5D, FIGS. 6A and 6B, FIGS. 7A to 7D, FIGS. 8A to 8F, FIGS. 9A to 9F, FIGS. 10A to 10F, FIGS. 11A to 11F, FIGS. 12A and 12B, FIG. 13, FIGS. 14A and 14B, FIGS. 15A and 15B, and FIG. 16.

1-1. Structural Example 1 of Semiconductor Device

FIG. 1A is a top view of a transistor 100 that is a semiconductor device of one embodiment of the present invention. FIG. 1B is a cross-sectional view taken along the dashed dotted line X1-X2 in FIG. 1A, and FIG. 1C is a cross-sectional view taken along the dashed dotted line Y1-Y2 in FIG. 1A. Note that in FIG. 1A, some components of the transistor 100 (e.g., an insulating film functioning as a gate insulating film) are not illustrated to avoid complexity. Furthermore, the direction of the dashed dotted line X1-X2 may be called a channel length direction, and the direction of the dashed dotted line Y1-Y2 may be called a channel width direction. As in FIG. 1A, some components are not illustrated in some cases in top views of transistors described below.

The transistor 100 includes a conductive film 104 functioning as a first gate electrode over a substrate 102, an insulating film 106 over the substrate 102 and the conductive film 104, an insulating film 107 over the insulating film 106, an oxide semiconductor film 108 over the insulating film 107, a conductive film 112 a functioning as a source electrode electrically connected to the oxide semiconductor film 108, a conductive film 112 b functioning as a drain electrode electrically connected to the oxide semiconductor film 108, insulating films 114 and 116 over the oxide semiconductor film 108 and the conductive films 112 a and 112 b, an oxide semiconductor film 120 a over the insulating film 116 and electrically connected to the conductive film 112 b, an oxide semiconductor film 120 b over the insulating film 116, and an insulating film 118 over the insulating film 116 and the oxide semiconductor film 120 a and 120 b.

In the transistor 100, the insulating films 106 and 107 function as a first gate insulating film of the transistor 100, the insulating films 114 and 116 function as a second gate insulating film of the transistor 100, and the insulating film 118 functions as a protective insulating film of the transistor 100. Note that in this specification and the like, in some cases, the insulating films 106 and 107 are collectively referred to as a first insulating film, the insulating films 114 and 116 are collectively referred to as a second insulating film, and the insulating film 118 is referred to as a third insulating film. In the transistor 100, the oxide semiconductor film 120 a functions as a pixel electrode of a display device, and the oxide semiconductor film 120 b functions as a second gate electrode of the transistor 100.

The oxide semiconductor film 108 includes an oxide semiconductor film 108 b and an oxide semiconductor film 108 c over the oxide semiconductor film 108 b. The oxide semiconductor film 108 b is closer to the conductive film 104 functioning as a first gate electrode than the oxide semiconductor film 108 c. The oxide semiconductor film 108 b and the oxide semiconductor film 108 c each include In, M (M is Al, Ga, Y, or Sn), and Zn.

The oxide semiconductor film 108 b preferably includes a region in which the atomic proportion of In is larger than the atomic proportion of M, for example. The oxide semiconductor film 108 c preferably includes a region in which the number of atoms of In is smaller than that in the oxide semiconductor film 108 b.

The oxide semiconductor film 108 b including the region in which the atomic proportion of In is larger than that of M can increase the field-effect mobility (also simply referred to as mobility or μFE) of the transistor 100. Specifically, the field-effect mobility of the transistor 100 can exceed 10 cm²/Vs, preferably exceed 30 cm²/Vs.

For example, the use of the transistor with high field-effect mobility for a gate driver that generates a gate signal (specifically, a demultiplexer connected to an output terminal of a shift register included in a gate driver) allows a semiconductor device or a display device to have a narrow frame.

On the other hand, the oxide semiconductor film 108 b including the region in which the atomic proportion of In is larger than that of M makes it easier to change electrical characteristics of the transistor 100 in light irradiation. However, in the semiconductor device of one embodiment of the present invention, the oxide semiconductor film 108 c is formed over the oxide semiconductor film 108 b. Furthermore, the oxide semiconductor film 108 c including the region in which the atomic proportion of In is smaller than the oxide semiconductor film 108 b has larger Eg than that of the oxide semiconductor film 108 b. For this reason, the oxide semiconductor film 108 which is a layered structure of the oxide semiconductor film 108 b and the oxide semiconductor film 108 c has high resistance to a negative bias stress test with light irradiation.

Impurities such as hydrogen or moisture entering the channel region of the oxide semiconductor film 108, particularly the oxide semiconductor film 108 b adversely affect the transistor characteristics and therefore cause a problem. Moreover, it is preferable that the amount of impurities such as hydrogen or moisture in the channel region of the oxide semiconductor film 108 b be as small as possible. Furthermore, oxygen vacancies formed in the channel region in the oxide semiconductor film 108 b adversely affect the transistor characteristics and therefore cause a problem. For example, oxygen vacancies formed in the channel region in the oxide semiconductor film 108 b are bonded to hydrogen to serve as a carrier supply source. The carrier supply source generated in the channel region in the oxide semiconductor film 108 b causes a change in the electrical characteristics, typically, shift in the threshold voltage, of the transistor 100 including the oxide semiconductor film 108 b. Therefore, it is preferable that the amount of oxygen vacancies in the channel region of the oxide semiconductor film 108 b be as small as possible.

In view of this, one embodiment of the present invention is a structure in which insulating films in contact with the oxide semiconductor film 108, specifically the insulating film 107 formed under the oxide semiconductor film 108 and the insulating films 114 and 116 formed over the oxide semiconductor film 108 include excess oxygen. Oxygen or excess oxygen is transferred from the insulating film 107 and the insulating films 114 and 116 to the oxide semiconductor film 108, whereby the oxygen vacancies in the oxide semiconductor film can be reduced. As a result, a change in electrical characteristics of the transistor 100, particularly a change in the transistor 100 due to light irradiation, can be reduced.

In one embodiment of the present invention, a manufacturing method is used in which the number of manufacturing steps is not increased or an increase in the number of manufacturing steps is extremely small, because the insulating film 107 and the insulating films 114 and 116 are made to contain excess oxygen. Thus, the transistors 100 can be manufactured with high yield.

Specifically, in a step of forming the oxide semiconductor film 108 b, the oxide semiconductor film 108 b is formed by a sputtering method in an atmosphere containing an oxygen gas, whereby oxygen or excess oxygen is added to the insulating film 107 over which the oxide semiconductor film 108 b is formed.

Furthermore, in a step of forming the oxide semiconductor films 120 a and 120 b, the oxide semiconductor films 120 a and 120 b are formed by a sputtering method in an atmosphere containing an oxygen gas, whereby oxygen or excess oxygen is added to the insulating film 116 over which the oxide semiconductor films 120 a and 120 b are formed. Note that in some cases, oxygen or excess oxygen is added also to the insulating film 114 and the oxide semiconductor film 108 under the insulating film 116 when oxygen or excess oxygen is added to the insulating film 116.

1-2. Amount of Oxygen Released from Insulating Films in Thermal Desorption Spectroscopy Method (TDS)

Described below are the measurement results of the amount of oxygen released from insulating films over which oxide semiconductor films were formed in an atmosphere containing an oxygen gas.

To measure the amount of oxygen released from the insulating films, Samples A1 to A20 to be described below were formed and the released amount of oxygen in TDS was evaluated.

[1-2-1. Evaluation for Determining the Amounts of Oxygen Added to Insulating Films by Varying Target Composition]

First, Samples A1 to A5 are described. Note that the target composition of Samples A1 to A5 is varied to determine the amounts of oxygen added to the insulating films.

(Sample A1)

As Sample A1, a 400-nm-thick silicon oxynitride film was formed over a glass substrate using a PECVD apparatus, and then, heat treatment was performed. The silicon oxynitride film was formed under the conditions where the substrate temperature was 350° C., a silane gas at a flow rate of 160 sccm and a dinitrogen monoxide gas at a flow rate of 4000 sccm were introduced into a chamber, the pressure was 200 Pa, and an RF power of 1500 W was supplied between parallel-plate electrodes provided in a PECVD apparatus. The heat treatment was performed at 650° C. in a nitrogen atmosphere for 6 minutes using an RTA apparatus. By the heat treatment, oxygen included in the silicon oxynitride film at the time of deposition is released from the silicon oxynitride film.

As each of Samples A2 to A5, a 400-nm-thick silicon oxynitride film was formed over a glass substrate using a PECVD apparatus, and then, heat treatment was performed. After the heat treatment, a 50-nm-thick oxide semiconductor film was formed over the silicon oxynitride film using a sputtering apparatus. The oxide semiconductor film was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 2500 W was supplied to a target provided in the sputtering apparatus. Note that the targets for forming the oxide semiconductor films of Samples A2 to A5 differ in composition. The composition of the targets for forming the oxide semiconductor films of Samples A2 to A5 is described below. After the oxide semiconductor film was formed, the oxide semiconductor film was removed to expose a surface of the silicon oxynitride film. Note that the formation conditions of the oxide semiconductor film, the formation conditions of the silicon oxynitride film, and the heat treatment conditions were the same as those of Sample A1.

(Sample A2)

The composition of the target for forming the oxide semiconductor film of Sample A2 was In:Ga:Zn=1:1:1.2 in an atomic ratio.

(Sample A3)

The composition of the target for forming the oxide semiconductor film of Sample A3 was In:Ga:Zn=4:2:4.1 in an atomic ratio.

(Sample A4)

The composition of the target for forming the oxide semiconductor film of Sample A4 was In:Ga:Zn=3:1:2 in an atomic ratio.

(Sample A5)

The composition of the target for forming the oxide semiconductor film of Sample A5 was In:Ga:Zn=1:3:6 in an atomic ratio.

Then, the amount of a gas having a mass-to-charge ratio (M/z) of 32, i.e., oxygen (O₂), released from each of the silicon oxynitride films of Samples A1 to A5 was measured. A TDS apparatus was used for measuring the amount of released gas. In a TDS apparatus, the released amount of a gas corresponding to oxygen was measured in a film-surface temperature range from 50° C. to 600° C.

FIG. 13 shows the TDS measurement results of Samples A1 to A5. In FIG. 13, the vertical axis indicates the released amount of a gas with M/z=32, and the horizontal axis indicates information on each sample such as a sample name.

As shown in the results in FIG. 13, the amount of a gas with M/z=32 released from Sample A1 was 1.82×10¹⁴/cm². The amount of a gas with M/z=32 released from Sample A2 was 1.22×10¹⁵/cm². The amount of a gas with M/z=32 released from Sample A3 was 1.14×10¹⁵/cm². The amount of a gas with M/z=32 released from Sample A4 was 1.18×10¹⁵/cm². The amount of a gas with M/z=32 released from Sample A5 was 1.20×10¹⁵/cm².

The above results suggest that excess oxygen can be added to the insulating film by forming the oxide semiconductor film by a sputtering method over the insulating film (here, the silicon oxynitride film) over which the oxide semiconductor film is to be formed.

Note that a large difference in the amount of oxygen added to the insulating film (oxygen released from the insulating film) due to the difference in the composition of the target for forming the oxide semiconductor film was not observed.

[1-2-2. Evaluation of the Amounts of Oxygen Added to Insulating Films by Varying Flow Rates of Deposition Gas]

Next, Samples A6 to A8 are described. Note that the flow rates of deposition gases for Samples A6 to A8 are varied to determine the amounts of oxygen added to the insulating films.

As each of Samples A6 to A8, a 400-nm-thick silicon oxynitride film was formed over a glass substrate using a PECVD apparatus, and then, heat treatment was performed. After the heat treatment, a 50-nm-thick oxide semiconductor film was formed over the silicon oxynitride film using a sputtering apparatus. Then, the oxide semiconductor film was removed to expose a surface of the silicon oxynitride film. Note that the formation conditions of the silicon oxynitride film and the heat treatment conditions were the same as those of Samples A1 to A5. The formation conditions of the oxide semiconductor films of Samples A6 to A8 are different from each other.

(Sample A6)

The oxide semiconductor film of Sample A6 was formed under the conditions where the substrate temperature was 170° C., an argon gas at a flow rate of 90 sccm and an oxygen gas at a flow rate of 10 sccm were introduced into a chamber, i.e., a 10% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 2500 W was supplied to a target (In:Ga:Zn=4:2:4.1) provided in a sputtering apparatus.

(Sample A7)

The oxide semiconductor film of Sample A7 was formed under the conditions where the substrate temperature was 170° C., an argon gas at a flow rate of 50 sccm and an oxygen gas at a flow rate of 50 sccm were introduced into a chamber, i.e., a 50% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 2500 W was supplied to a target (In:Ga:Zn=4:2:4.1) provided in a sputtering apparatus.

(Sample A8)

The oxide semiconductor film of Sample A8 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 2500 W was supplied to a target (In:Ga:Zn=4:2:4.1) provided in a sputtering apparatus. Note that Sample A8 is the same as Sample A3.

Then, the amount of a gas having a mass-to-charge ratio (M/z) of 32, i.e., oxygen (O₂), released from the silicon oxynitride film of each of Samples A6 to A8 was measured. A TDS analysis apparatus was used for measuring the amount of released gas.

FIG. 14A shows the TDS measurement results of Samples A6 to A8. In FIG. 14A, the vertical axis indicates the released amount of a gas with M/z=32, and the horizontal axis indicates information on each sample such as a sample name.

As shown in the results in FIG. 14A, the amount of a gas with M/z=32 released from Sample A6 was 1.19×10¹⁴/cm². The amount of a gas with M/z=32 released from Sample A7 was 5.02×10¹⁴/cm². The amount of a gas with M/z=32 released from Sample A8 was 1.14×10¹⁵/cm².

The above results show that the flow rate of an oxygen gas in the formation conditions of the oxide semiconductor film is preferably higher in the case where excess oxygen is added to the insulating film when the oxide semiconductor film is formed by a sputtering method over the insulating film (here, the silicon oxynitride film) over which the oxide semiconductor film is to be formed.

[1-2-3. Evaluation of the Amounts of Oxygen Added to Insulating Films by Varying Deposition Powers]

Next, Samples A9 to A12 are described. Note that the deposition powers for Samples A9 to A12 are varied to determine the amounts of oxygen added to the insulating films.

As each of Samples A9 to A12, a 400-nm-thick silicon oxynitride film was formed over a glass substrate using a PECVD apparatus, and then, heat treatment was performed. After the heat treatment, a 50-nm-thick oxide semiconductor film was formed over the silicon oxynitride film using a sputtering apparatus. Then, the oxide semiconductor film was removed to expose a surface of the silicon oxynitride film. Note that the formation conditions of the silicon oxynitride film and the heat treatment conditions were the same as those of Samples A1 to A8. The formation conditions of the oxide semiconductor films of Samples A9 to A12 are different from each other.

(Sample A9)

The oxide semiconductor film of Sample A9 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 500 W was supplied to a target (In:Ga:Zn=4:2:4.1) provided in a sputtering apparatus.

(Sample A10)

The oxide semiconductor film of Sample A10 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 1500 W was supplied to a target (In:Ga:Zn=4:2:4.1) provided in a sputtering apparatus.

(Sample A11)

The oxide semiconductor film of Sample A11 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 2500 W was supplied to a target (In:Ga:Zn=4:2:4.1) provided in a sputtering apparatus. Note that Sample A11 is the same as Samples A3 and A8.

(Sample A12)

The oxide semiconductor film of Sample A12 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 4500 W was supplied to a target (In:Ga:Zn=4:2:4.1) provided in a sputtering apparatus.

Then, the amount of a gas having a mass-to-charge ratio (M/z) of 32, i.e., oxygen (O₂), released from the silicon oxynitride film of each of Samples A9 to A12 was measured. A TDS analysis apparatus was used for measuring the amount of released gas.

FIG. 14B shows the TDS measurement results of Samples A9 to A12. In FIG. 14B, the vertical axis indicates the released amount of a gas with M/z=32, and the horizontal axis indicates information on each sample such as a sample name.

As shown in the results in FIG. 14B, the amount of a gas with M/z=32 released from Sample A9 was 9.28×10¹⁴/cm². The amount of a gas with M/z=32 released from Sample A10 was 1.07×10¹⁵/cm². The amount of a gas with M/z=32 released from Sample A11 was 1.14×10¹⁵/cm². The amount of a gas with M/z=32 released from Sample A12 was 1.24×10¹⁵/cm².

The above results show that the deposition power in the formation conditions of the oxide semiconductor film is preferably higher in the case where excess oxygen is added to the insulating film when the oxide semiconductor film is formed by a sputtering method over the insulating film (here, the silicon oxynitride film) over which the oxide semiconductor film is to be formed.

[1-2-4. Evaluation of the Amounts of Oxygen Added to Insulating Films by Varying Deposition Pressures]

Next, Samples A13 to A15 are described. Note that the deposition pressures for Samples A13 to A15 are varied to determine the amounts of oxygen added to the insulating films.

As each of Samples A13 to A15, a 400-nm-thick silicon oxynitride film was formed over a glass substrate using a PECVD apparatus, and then, heat treatment was performed. After the heat treatment, a 50-nm-thick oxide semiconductor film was formed over the silicon oxynitride film using a sputtering apparatus. Then, the oxide semiconductor film was removed to expose a surface of the silicon oxynitride film. Note that the formation conditions of the silicon oxynitride film and the heat treatment conditions were the same as those of Samples A1 to A12. The formation conditions of the oxide semiconductor films of Samples A13 to A15 are different from each other.

(Sample A13)

The oxide semiconductor film of Sample A13 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.3 Pa, and an AC power of 2500 W was supplied to a target (In:Ga:Zn=4:2:4.1) provided in a sputtering apparatus.

(Sample A14)

The oxide semiconductor film of Sample A14 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 2500 W was supplied to a target (In:Ga:Zn=4:2:4.1) provided in a sputtering apparatus. Note that Sample A14 is the same as Samples A3, A8, and A11.

(Sample A15)

The oxide semiconductor film of Sample A15 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.8 Pa, and an AC power of 2500 W was supplied to a target (In:Ga:Zn=4:2:4.1) provided in a sputtering apparatus.

Then, the amount of a gas having a mass-to-charge ratio (M/z) of 32, i.e., oxygen (O₂), released from the silicon oxynitride film of each of Samples A13 to A15 was measured. A TDS analysis apparatus was used for measuring the amount of released gas.

FIG. 15A shows the TDS measurement results of Samples A13 to A15. In FIG. 15A, the vertical axis indicates the released amount of a gas with M/z=32, and the horizontal axis indicates information on each sample such as a sample name.

As shown in the results in FIG. 15A, the amount of a gas with M/z=32 released from Sample A13 was 1.17×10¹⁵/cm². The amount of a gas with M/z=32 released from Sample A14 was 1.14×10¹⁵/cm². The amount of a gas with M/z=32 released from Sample A15 was 1.15×10¹⁵/cm².

The above results show that the difference in the amount of oxygen added to the insulating film (oxygen released from the insulating film) due to the difference in the pressure for forming the oxide semiconductor film was not observed.

[1-2-5. Evaluation of the Amounts of Oxygen Added to Insulating Films by Varying Thicknesses]

Next, Samples A16 to A20 are described. Note that in Samples A16 to A20, thicknesses are varied to determine the amounts of oxygen added to the insulating films.

As each of Samples A16 to A20, a 400-nm-thick silicon oxynitride film was formed over a glass substrate using a PECVD apparatus, and then, heat treatment was performed. After the heat treatment, an oxide semiconductor film was formed over the silicon oxynitride film using a sputtering apparatus. Then, the oxide semiconductor film was removed to expose a surface of the silicon oxynitride film. Note that the formation conditions of the silicon oxynitride film and the heat treatment conditions were the same as those of Samples A1 to A15. The thicknesses of the oxide semiconductor films of Samples A16 to A20 are different from each other.

(Sample A16)

The film thickness of the oxide semiconductor film of Sample A16 was 0 nm. That is, the oxide semiconductor film was not formed in Sample A16. Note that Sample A16 is the same as Sample A1.

(Sample A17)

The thickness of the oxide semiconductor film of Sample A17 was 10 nm. The oxide semiconductor film of Sample A17 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 2500 W was supplied to a target (In:Ga:Zn=4:2:4.1 [atomic ratio]) provided in a sputtering apparatus.

(Sample A18)

The thickness of the oxide semiconductor film of Sample A18 was 50 nm. Note that the formation conditions of the oxide semiconductor film of Sample A18 were the same as those of Sample A17. Note that Sample A18 is the same as Samples A3, A8, A11, and A14.

(Sample A19)

The thickness of the oxide semiconductor film of Sample A19 was 100 nm. Note that the formation conditions of the oxide semiconductor film of Sample A19 were the same as those of Sample A17.

(Sample A20)

The thickness of the oxide semiconductor film of Sample A20 was 300 nm. Note that the formation conditions of the oxide semiconductor film of Sample A20 were the same as those of Sample A17.

Then, the amount of a gas having a mass-to-charge ratio (M/z) of 32, i.e., oxygen (O₂), released from the silicon oxynitride film of each of Samples A16 to A20 was measured. A TDS analysis apparatus was used for measuring the amount of released gas.

FIG. 15B shows the TDS measurement results of Samples A16 to A20. In FIG. 15B, the vertical axis indicates the released amount of a gas with M/z=32, and the horizontal axis indicates information on each sample such as a sample name.

As shown in the results in FIG. 15B, the amount of a gas with M/z=32 released from Sample A16 was 1.82×10¹⁴/cm². The amount of a gas with M/z=32 released from Sample A17 was 1.08×10¹⁵/cm². The amount of a gas with M/z=32 released from Sample A18 was 1.14×10¹⁵/cm². The amount of a gas with M/z=32 released from Sample A19 was 1.11×10¹⁵/cm². The amount of a gas with M/z=32 released from Sample A20 was 1.17×10¹⁵/cm².

The above results suggest that excess oxygen can be added to the insulating film by forming the oxide semiconductor film to a thickness of 10 nm or more by a sputtering method over the insulating film (here, the silicon oxynitride film) over which the oxide semiconductor film is to be formed.

1-3. Oxide Conductor

Next, an oxide conductor is described. In a step of forming the oxide semiconductor films 120 a and 120 b, the oxide semiconductor films 120 a and 120 b serve as a protective film for suppressing release of oxygen from the insulating films 114 and 116. The oxide semiconductor films 120 a and 120 b serve as semiconductors before a step of forming the insulating film 118 and serve as conductors after the step of forming the insulating film 118. Therefore, the oxide semiconductor films 120 a and 120 b may each be referred to as oxide conductor (OC).

To allow the oxide semiconductor films 120 a and 120 b to serve as conductors, an oxygen vacancy is formed in the oxide semiconductor films 120 a and 120 b and hydrogen is added from the insulating film 118 to the oxygen vacancy, whereby a donor level is formed in the vicinity of the conduction band. As a result, the conductivity of each of the oxide semiconductor films 120 a and 120 b is increased, so that the oxide semiconductor film becomes a conductor. The oxide semiconductor films 120 a and 120 b having become conductors can each be referred to as oxide conductor. Oxide semiconductors generally have a visible light transmitting property because of their large energy gap. An oxide conductor is an oxide semiconductor having a donor level in the vicinity of the conduction band. Therefore, the influence of absorption due to the donor level is small in an oxide conductor, and an oxide conductor has a visible light transmitting property comparable to that of an oxide semiconductor.

The temperature dependence of the resistivity of a film formed using an oxide conductor (hereinafter referred to as an oxide conductor film) will be described with reference to FIG. 16.

Samples B1 to B4 each including an oxide conductor film were formed. As each of Samples B1 to B3, a 100-nm-thick oxide semiconductor film was formed over a glass substrate by a sputtering method. Then, a 100-nm-thick silicon nitride film containing hydrogen was formed over the oxide semiconductor film by a PECVD method. Note that Samples B1 to B3 differ in the target composition of the oxide semiconductor film and the formation conditions of the oxide semiconductor film. As Sample B4, a 100-nm-thick oxide conductor film was formed over a glass substrate by a sputtering method.

FIG. 16 shows measured sheet resistivity of each sample. Here, the sheet resistivity was measured by a four-point van der Pauw method. In FIG. 16, the vertical axis indicates sheet resistivity, and horizontal axis indicates measurement temperature. Methods for forming the samples are described below.

(Sample B1)

The oxide semiconductor film of Sample B1 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 2500 W was supplied to a target (In:Ga:Zn=4:2:4.1 [atomic ratio]) provided in the sputtering apparatus. The oxide semiconductor film of Sample B1 is denoted by IGZO(423) in FIG. 16. Note that numerical values in parentheses refer to the composition of a formed film, not the composition of the target; the same applies to IGZO(312) and IGZO(136).

(Sample B2)

The oxide semiconductor film of Sample B2 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.6 Pa, and an AC power of 2500 W was supplied to a target (In:Ga:Zn=3:1:2 [atomic ratio]) provided in the sputtering apparatus. The oxide semiconductor film of Sample B2 is denoted by IGZO(312) in FIG. 16.

(Sample B3)

The oxide semiconductor film of Sample B3 was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, i.e., a 100% oxygen gas atmosphere was used, the pressure was 0.8 Pa, and an AC power of 2500 W was supplied to a target (In:Ga:Zn=1:3:6) provided in the sputtering apparatus. The oxide semiconductor film of Sample B3 is denoted by IGZO(136) in FIG. 16.

(Sample B4)

The oxide conductor film of Sample B4 was formed under the conditions where the substrate temperature was room temperature, an argon gas at a flow rate of 72 sccm and an oxygen gas at a flow rate of 5 sccm were introduced into a chamber, the pressure was 0.15 Pa, and a DC power of 3200 W was supplied to a target (an oxide target containing indium, tin, and silicon; In₂O₃:SnO₂:SiO₂=85:10:5 [wt. %]) provided in a sputtering apparatus. The oxide conductor film of Sample B4 is denoted by NITO in FIG. 16.

The results in FIG. 16 show that the sheet resistivity of the oxide semiconductor film in each of Sample B1 and B2 is higher than that of Sample B4 and is higher than or equal to 1.0×10² Ω/sq. and lower than or equal to 1.0×10³ Ω/sq. Moreover, a change in sheet resistivity depending on the measurement temperature is extremely small. In other words, the oxide conductor in each of Samples B1 and B2 is a degenerate semiconductor and it is suggested that the conduction band minimum agrees with or substantially agrees with the Fermi level. Meanwhile, the oxide semiconductor film of Sample B3 had a high sheet resistivity beyond the limit of measurement (higher than or equal to 1.0×10⁶ Ω/sq.) and therefore the sheet resistivity was difficult to measure.

As described above, in a step of forming an oxide semiconductor film in the semiconductor device of one embodiment of the present invention, oxygen is added to a surface over which the oxide semiconductor film is to be formed. Furthermore, a first oxide semiconductor film functioning as a channel region and a second oxide semiconductor film functioning as a second gate electrode contain the same metal element, and an insulating film containing hydrogen is formed over the second oxide semiconductor film, whereby the second oxide semiconductor film becomes an oxide conductor film. The use of such a structure can achieve a highly reliable semiconductor device in which the variation in electrical characteristics is suppressed.

1-4. Constituent Elements of Semiconductor Device

Constituent elements of the semiconductor device of this embodiment will be described below in detail.

[Substrate]

There is no particular limitation on the property of a material and the like of the substrate 102 as long as the material has heat resistance enough to withstand at least heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like may be used as the substrate 102. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used as the substrate 102. Still alternatively, any of these substrates provided with a semiconductor element may be used as the substrate 102. In the case where a glass substrate is used as the substrate 102, a glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be manufactured.

Alternatively, a flexible substrate may be used as the substrate 102, and the transistor 100 may be provided directly on the flexible substrate. Alternatively, a separation layer may be provided between the substrate 102 and the transistor 100. The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate 102 and transferred onto another substrate. In such a case, the transistor 100 can be transferred to a substrate having low heat resistance or a flexible substrate as well.

[Conductive Film Functioning as a First Gate Electrode and Source and Drain Electrodes]

The conductive film 104 functioning as a gate electrode and the conductive films 112 a and 112 b functioning as a source electrode and a drain electrode, respectively, can each be formed using a metal element selected from chromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc (Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloy including any of these metal element as its component; an alloy including a combination of any of these metal elements; or the like.

Furthermore, the conductive films 104, 112 a, and 112 b may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. Alternatively, an alloy film or a nitride film in which aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium are combined may be used.

The conductive films 104, 112 a, and 112 b can be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added.

A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be used for the conductive films 104, 112 a, and 112 b. Use of a Cu—X alloy film enables the manufacturing cost to be reduced because wet etching process can be used in the processing.

[Insulating Films Functioning as First Gate Insulating Film]

As each of the insulating films 106 and 107 functioning as gate insulating films of the transistor 100, an insulating layer including at least one of the following films formed by a plasma enhanced chemical vapor deposition (PECVD) method, a sputtering method, or the like can be used: a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, and a neodymium oxide film. Note that instead of a stacked-layer structure of the insulating films 106 and 107, an insulating film of a single layer formed using a material selected from the above or an insulating film of three or more layers may be used.

The insulating film 106 has a function as a blocking film which inhibits penetration of oxygen. For example, in the case where excess oxygen is supplied to the insulating film 107, the insulating film 114, the insulating film 116, and/or the oxide semiconductor film 108, the insulating film 106 can inhibit penetration of oxygen.

Note that the insulating film 107 that is in contact with the oxide semiconductor film 108 functioning as a channel region of the transistor 100 is preferably an oxide insulating film and preferably includes a region including oxygen in excess of the stoichiometric composition (oxygen-excess region). In other words, the insulating film 107 is an insulating film capable of releasing oxygen. In order to provide the oxygen excess region in the insulating film 107, the insulating film 107 is formed in an oxygen atmosphere, for example. Alternatively, oxygen may be added to the deposited insulating film 107. A method for adding oxygen to the deposited insulating film 107 is described later.

In the case where hafnium oxide is used for the insulating film 107, the following effect is attained. Hafnium oxide has a higher dielectric constant than silicon oxide and silicon oxynitride. Therefore, by using hafnium oxide, the thickness of the insulating film 107 can be made large as compared with the case where silicon oxide is used; thus, leakage current due to tunnel current can be low. That is, it is possible to provide a transistor with a low off-state current. Moreover, hafnium oxide with a crystalline structure has higher dielectric constant than hafnium oxide with an amorphous structure. Therefore, it is preferable to use hafnium oxide with a crystalline structure in order to provide a transistor with a low off-state current. Examples of the crystalline structure include a monoclinic crystal structure and a cubic crystal structure. Note that one embodiment of the present invention is not limited thereto.

In this embodiment, a silicon nitride film is formed as the insulating film 106, and a silicon oxide film is formed as the insulating film 107. The silicon nitride film has a higher dielectric constant than a silicon oxide film and needs a larger thickness for capacitance equivalent to that of the silicon oxide film. Thus, when the silicon nitride film is included in the gate insulating film of the transistor 150, the physical thickness of the insulating film can be increased. This makes it possible to reduce a decrease in withstand voltage of the transistor 100 and furthermore to increase the withstand voltage, thereby reducing electrostatic discharge damage to the transistor 100.

[Oxide Semiconductor Film]

The oxide semiconductor film 108 can be formed using the materials described above.

In the case where the oxide semiconductor film 108 b is In-M-Zn oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming a film of the In-M-Zn oxide satisfy In>M. Examples of the atomic ratio of metal elements of such a sputtering target include In:M:Zn=2:1:3, In:M:Zn=3:1:2, and In:M:Zn=4:2:4.1.

In the case where the oxide semiconductor film 108 c is In-M-Zn oxide, the atomic ratio between metal elements in a sputtering target used for forming the In-M-Zn oxide preferably satisfies In≦M. The atomic ratio between metal elements in such a sputtering target is In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:6, or the like.

In the case where the oxide semiconductor film 108 b and the oxide semiconductor film 108 c are each In-M-Zn oxide, it is preferable to use a target including polycrystalline In-M-Zn oxide as the sputtering target. The use of the target including polycrystalline In-M-Zn oxide facilitates formation of the oxide semiconductor film 108 b and the oxide semiconductor film 108 c having crystallinity. Note that the atomic ratios of metal elements in the oxide semiconductor film 108 b and the oxide semiconductor film 108 c vary from the above atomic ratio of metal elements of the sputtering target within a range of ±40%. For example, when a sputtering target with an atomic ratio of In to Ga and Zn of 4:2:4.1 is used for forming the oxide semiconductor film 108 b, the atomic ratio of In to Ga and Zn in the oxide semiconductor film 108 b may be 4:2:3 or in the vicinity of 4:2:3.

The energy gap of the oxide semiconductor film 108 is 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more. The use of an oxide semiconductor having a wide energy gap can reduce off-state current of the transistor 100. In particular, an oxide semiconductor film having an energy gap more than or equal to 2 eV, preferably more than or equal to 2 eV and less than or equal to 3.0 eV is preferably used as the oxide semiconductor film 108 b, and an oxide semiconductor film having an energy gap more than or equal to 2.5 eV and less than or equal to 3.5 eV is preferably used as the oxide semiconductor film 108 c. Furthermore, the oxide semiconductor film 108 c preferably has a higher energy gap than that of the oxide semiconductor film 108 b.

Each thickness of the oxide semiconductor film 108 b and the oxide semiconductor film 108 c is more than or equal to 3 nm and less than or equal to 200 nm, preferably more than or equal to 3 nm and less than or equal to 100 nm, more preferably more than or equal to 3 nm and less than or equal to 50 nm.

An oxide semiconductor film with low carrier density is used as the oxide semiconductor film 108 c. For example, the carrier density of the second oxide semiconductor film 108 c is lower than or equal to 1×10¹⁷/cm³, preferably lower than or equal to 1×10¹⁵/cm³, further preferably lower than or equal to 1×10¹³/cm³, still further preferably lower than or equal to 1×10¹¹/cm³.

Note that, without limitation to the compositions and materials described above, a material with an appropriate composition may be used depending on required semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of a transistor. Further, in order to obtain required semiconductor characteristics of a transistor, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like of the oxide semiconductor film 108 b and the oxide semiconductor film 108 c be set to be appropriate.

Note that it is preferable to use, as the oxide semiconductor film 108 b and the oxide semiconductor film 108 c, an oxide semiconductor film in which the impurity concentration is low and the density of defect states is low, in which case the transistor can have more excellent electrical characteristics. Here, the state in which the impurity concentration is low and the density of defect states is low (the amount of oxygen vacancy is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor in which a channel region is formed in the oxide semiconductor film rarely has a negative threshold voltage (is rarely normally on). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has a low density of trap states in some cases. Further, the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; even when an element has a channel width W of 1×10⁶ μm and a channel length L of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, that is, less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V.

Accordingly, the transistor in which the channel region is formed in the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film can have a small change in electrical characteristics and high reliability. Charges trapped by the trap states in the oxide semiconductor film take a long time to be released and may behave like fixed charges. Thus, the transistor whose channel region is formed in the oxide semiconductor film having a high density of trap states has unstable electrical characteristics in some cases. As examples of the impurities, hydrogen, nitrogen, alkali metal, alkaline earth metal, and the like are given.

Hydrogen included in the oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and also causes oxygen vacancy in a lattice from which oxygen is released (or a portion from which oxygen is released). Due to entry of hydrogen into the oxygen vacancy, an electron serving as a carrier is generated in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor film which contains hydrogen is likely to be normally on. Accordingly, it is preferable that hydrogen be reduced as much as possible in the oxide semiconductor film 108. Specifically, in the oxide semiconductor film 108, the concentration of hydrogen which is measured by SIMS is lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, further preferably lower than or equal to 1×10¹⁹ atoms/cm³, further preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³, further preferably lower than or equal to 5×10¹⁷ atoms/cm³, and further preferably lower than or equal to 1×10¹⁶ atoms/cm³.

The oxide semiconductor film 108 b preferably includes a region in which hydrogen concentration is smaller than that in the oxide semiconductor film 108 c. A semiconductor device including the oxide semiconductor film 108 b having the region in which hydrogen concentration is smaller than that in the oxide semiconductor film 108 c can be increased in reliability.

When silicon or carbon that is one of elements belonging to Group 14 is included in the oxide semiconductor film 108 b, oxygen vacancy is increased in the oxide semiconductor film 108 b, and the oxide semiconductor film 108 b becomes an n-type film. Thus, the concentration of silicon or carbon (the concentration is measured by SIMS) in the oxide semiconductor film 108 b or the concentration of silicon or carbon (the concentration is measured by SIMS) in the vicinity of an interface with the oxide semiconductor film 108 b is set to be lower than or equal to 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³.

In addition, the concentration of alkali metal or alkaline earth metal of the oxide semiconductor film 108 b, which is measured by SIMS, is lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, in which case the off-state current of the transistor might be increased. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal of the oxide semiconductor film 108 b.

Furthermore, when including nitrogen, the oxide semiconductor film 108 b easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor including an oxide semiconductor film which contains nitrogen is likely to have normally-on characteristics. For this reason, nitrogen in the oxide semiconductor film is preferably reduced as much as possible; the concentration of nitrogen which is measured by SIMS is preferably set to be, for example, lower than or equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 108 b and the oxide semiconductor film 108 c may have a non-single-crystal structure, for example. The non-single crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure described later, or an amorphous structure, for example. Among the non-single crystal structure, the amorphous structure has the highest density of defect states, whereas CAAC-OS has the lowest density of defect states.

[Insulating Films Functioning as Second Gate Insulating Film]

The insulating films 114 and 116 function as a second gate insulating film of the transistor 100. In addition, the insulating films 114 and 116 each have a function of supplying oxygen to the oxide semiconductor film 108. That is, the insulating films 114 and 116 contain oxygen. Furthermore, the insulating film 114 is an insulating film which can transmit oxygen. Note that the insulating film 114 also functions as a film which relieves damage to the oxide semiconductor film 108 at the time of forming the insulating film 116 in a later step.

A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 5 nm and less than or equal to 150 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm can be used as the insulating film 114.

In addition, it is preferable that the number of defects in the insulating film 114 be small and typically, the spin density corresponding to a signal that appears at g=2.001 due to a dangling bond of silicon be lower than or equal to 3×10¹⁷ spins/cm³ by electron spin resonance (ESR) measurement. This is because if the density of defects in the insulating film 114 is high, oxygen is bonded to the defects and the amount of oxygen that transmits the insulating film 114 is decreased.

Note that all oxygen entering the insulating film 114 from the outside does not move to the outside of the insulating film 114 and some oxygen remains in the insulating film 114. Furthermore, movement of oxygen occurs in the insulating film 114 in some cases in such a manner that oxygen enters the insulating film 114 and oxygen included in the insulating film 114 moves to the outside of the insulating film 114. When an oxide insulating film which can transmit oxygen is formed as the insulating film 114, oxygen released from the insulating film 116 provided over the insulating film 114 can be moved to the oxide semiconductor film 108 through the insulating film 114.

Note that the insulating film 114 can be formed using an oxide insulating film having a low density of states due to nitrogen oxide. Note that the density of states due to nitrogen oxide can be formed between the energy of the valence band maximum (E_(v) _(_) _(os)) and the energy of the conduction band minimum (E_(c) _(_) _(os)) of the oxide semiconductor film. A silicon oxynitride film that releases less nitrogen oxide, an aluminum oxynitride film that releases less nitrogen oxide, and the like can be used as the above oxide insulating film.

Note that a silicon oxynitride film that releases less nitrogen oxide is a film of which the amount of released ammonia is larger than the amount of released nitrogen oxide in TDS analysis; the amount of released ammonia is typically greater than or equal to 1×10¹⁸/cm³ and less than or equal to 5×10¹⁹/cm³. Note that the amount of released ammonia is the amount of ammonia released by heat treatment with which the surface temperature of a film becomes higher than or equal to 50° C. and lower than or equal to 650° C., preferably higher than or equal to 50° C. and lower than or equal to 550° C.

Nitrogen oxide (NO_(x); x is greater than 0 and less than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2), typically NO₂ or NO, forms levels in the insulating film 114, for example. The level is positioned in the energy gap of the oxide semiconductor film 108. Therefore, when nitrogen oxide is diffused to the vicinity of the interface between the insulating film 114 and the oxide semiconductor film 108, an electron is in some cases trapped by the level on the insulating film 114 side. As a result, the trapped electron remains in the vicinity of the interface between the insulating film 114 and the oxide semiconductor film 108; thus, the threshold voltage of the transistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Since nitrogen oxide included in the insulating film 114 reacts with ammonia included in the insulating film 116 in heat treatment, nitrogen oxide included in the insulating film 114 is reduced. Therefore, an electron is hardly trapped at the vicinity of the interface between the insulating film 114 and the oxide semiconductor film 108.

By using such an oxide insulating film, the insulating film 114 can reduce the shift in the threshold voltage of the transistor, which leads to a smaller change in the electrical characteristics of the transistor.

Note that in an ESR spectrum at 100 K or lower of the insulating film 114, by heat treatment of a manufacturing process of the transistor, typically heat treatment at a temperature higher than or equal to 300° C. and lower than 350° C., a first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, a second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and a third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 are observed. The split width of the first and second signals and the split width of the second and third signals that are obtained by ESR measurement using an X-band are each approximately 5 mT. The sum of the spin densities of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is lower than 1×10¹⁸ spins/cm³, typically higher than or equal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 correspond to signals attributed to nitrogen oxide (NO_(x); x is greater than 0 and less than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2). Typical examples of nitrogen oxide include nitrogen monoxide and nitrogen dioxide. In other words, the lower the total spin density of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is, the lower the content of nitrogen oxide in the oxide insulating film is.

The concentration of nitrogen of the above oxide insulating film measured by SIMS is lower than or equal to 6×10²⁰ atoms/cm³.

The above oxide insulating film is formed by a PECVD method at a substrate temperature higher than or equal to 220° C. and lower than or equal to 350° C. using silane and dinitrogen monoxide, whereby a dense and hard film can be formed.

The insulating film 116 is formed using an oxide insulating film that contains oxygen in excess of that in the stoichiometric composition. Part of oxygen is released by heating from the oxide insulating film including oxygen in excess of that in the stoichiometric composition. The oxide insulating film including oxygen in excess of that in the stoichiometric composition is an oxide insulating film of which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10¹⁹ atoms/cm³, preferably greater than or equal to 3.0×10²⁰ atoms/cm³ in TDS analysis. Note that the temperature of the film surface in the TDS analysis is preferably higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 30 nm and less than or equal to 500 nm, preferably greater than or equal to 50 nm and less than or equal to 400 nm can be used as the insulating film 116.

It is preferable that the number of defects in the insulating film 116 be small, and typically the spin density corresponding to a signal which appears at g=2.001 due to a dangling bond of silicon be lower than 1.5×10¹⁸ spins/cm³, preferably lower than or equal to 1×10¹⁸ spins/cm³ by ESR measurement. Note that the insulating film 116 is provided more apart from the oxide semiconductor film 108 than the insulating film 114 is; thus, the insulating film 116 may have higher density of defects than the insulating film 114.

Furthermore, the insulating films 114 and 116 can be formed using insulating films formed of the same kinds of materials; thus, a boundary between the insulating films 114 and 116 cannot be clearly observed in some cases. Thus, in this embodiment, the boundary between the insulating films 114 and 116 is shown by a dashed line. Although a two-layer structure of the insulating films 114 and 116 is described in this embodiment, the present invention is not limited to this. For example, a single-layer structure of the insulating film 114 may be employed.

[Oxide Semiconductor Film Functioning as Pixel Electrode and Oxide Semiconductor Film Functioning as Second Gate Electrode]

The oxide semiconductor film 120 a functioning as a pixel electrode and the oxide semiconductor film 120 b functioning as a second gate electrode can be formed using a material and a formation method which are similar to those of the oxide semiconductor film 108.

The oxide semiconductor film 120 a functioning as a pixel electrode and the oxide semiconductor film 120 b functioning as a second gate electrode contain at least one metal element which is the same as that contained in the above-described oxide semiconductor film 108. For example, In oxide, In—Sn oxide, In—Zn oxide, In—Ga oxide, Zn oxide, Al—Zn oxide, In—Ga—Zn oxide, or the like can be used for the oxide semiconductor films 120 a and 120 b. It is particularly preferable to use an In—Sn oxide or an In—Ga—Zn oxide.

Specifically, a light-transmitting conductive material such as indium gallium zinc oxide (IGZO), indium tin oxide (ITO), indium zinc oxide, or indium tin silicon oxide (ITSO) can be used for the oxide semiconductor films 120 a and 120 b.

That is, the oxide semiconductor film 120 a functioning as a pixel electrode and the oxide semiconductor film 120 b functioning as a second gate electrode contain at least one metal element which is the same as that contained in the oxide semiconductor film 108 (the oxide semiconductor film 108 b and the oxide semiconductor film 108 c). For example, the oxide semiconductor film 120 b functioning as a second gate electrode and the oxide semiconductor film 108 (the oxide semiconductor film 108 b and the oxide semiconductor film 108 c) contain the same metal element; thus, the manufacturing cost can be reduced.

For example, in the case where the oxide semiconductor film 120 a functioning as a pixel electrode and the oxide semiconductor film 120 b functioning as a second gate electrode are each In-M-Zn oxide, the atomic ratio between metal elements in a sputtering target used for forming the In-M-Zn oxide preferably satisfies In≧M. The atomic ratio between metal elements in such a sputtering target is In:M:Zn=2:1:3, In:M:Zn=3:1:2, In:M:Zn=4:2:4.1, or the like.

The oxide semiconductor film 120 a functioning as a pixel electrode and the oxide semiconductor film 120 b functioning as a second gate electrode can each have a single-layer structure or a stacked-layer structure of two or more layers. Note that in the case where the oxide semiconductor film 120 a and the oxide semiconductor film 120 b each have a stacked-layer structure, the composition of the sputtering target is not limited to that described above. The case where the oxide semiconductor film 120 a and the oxide semiconductor film 120 b each have a stacked-layer structure will be described later.

[Insulating Film Functioning as Protective Insulating Film of Transistor]

The insulating film 118 serves as a protective insulating film of the transistor 100.

The insulating film 118 includes one or both of hydrogen and nitrogen. Alternatively, the insulating film 118 includes nitrogen and silicon. The insulating film 118 has a function of blocking oxygen, hydrogen, water, alkali metal, alkaline earth metal, or the like. It is possible to prevent outward diffusion of oxygen from the oxide semiconductor film 108, outward diffusion of oxygen included in the insulating films 114 and 116, and entry of hydrogen, water, or the like into the oxide semiconductor film 108 from the outside by providing the insulating film 118.

The insulating film 118 has a function of supplying one of or both hydrogen and nitrogen to the oxide semiconductor film 120 a functioning as a pixel electrode and the oxide semiconductor film 120 b functioning as a second gate electrode. The insulating film 118 preferably includes hydrogen and has a function of supplying the hydrogen to the oxide semiconductor films 120 a and 120 b. The oxide semiconductor films 120 a and 120 b supplied with hydrogen from the insulating film 118 function as conductors.

A nitride insulating film, for example, can be used as the insulating film 118. The nitride insulating film is formed using silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or the like.

Although the variety of films such as the conductive films, the insulating films, and the oxide semiconductor films which are described above can be formed by a sputtering method or a PECVD method, such films may be formed by another method, e.g., a thermal CVD method. Examples of the thermal CVD method include a metal organic chemical vapor deposition (MOCVD) method and an atomic layer deposition (ALD) method.

A thermal CVD method has an advantage that no defect due to plasma damage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a manner that a source gas and an oxidizer are supplied to the chamber at a time so that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and react with each other in the vicinity of the substrate or over the substrate.

Deposition by an ALD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching respective switching valves (also referred to as high-speed valves). For example, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time as or after the introduction of the first gas so that the source gases are not mixed, and then a second source gas is introduced. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first layer; then the second source gas is introduced to react with the first layer; as a result, a second layer is stacked over the first layer, so that a thin film is formed. The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute FET.

The variety of films such as the conductive films, the insulating films, the oxide semiconductor films, and the metal oxide films in this embodiment can be formed by a thermal CVD method such as an MOCVD method or an ALD method. For example, in the case where an In—Ga—Zn—O film is formed, trimethylindium, trimethylgallium, and dimethylzinc are used. Note that the chemical formula of trimethylindium is In(CH₃)₃. The chemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formula of dimethylzinc is Zn(CH₃)₂. Without limitation to the above combination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be used instead of trimethylgallium and diethylzinc (chemical formula: Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, that is, ozone (O₃) as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and a hafnium precursor compound (e.g., a hafnium alkoxide or a hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH)) are used. Note that the chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of another material liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, e.g., H₂O as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and an aluminum precursor compound (e.g., trimethylaluminum (TMA)) are used. Note that the chemical formula of trimethylaluminum is Al(CH₃)₃. Examples of another material liquid include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by a deposition apparatus using an ALD method, hexachlorodisilane is adsorbed on a surface where a film is to be formed, chlorine included in the adsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ or dinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using a deposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas are sequentially introduced plural times to form an initial tungsten film, and then a WF₆ gas and an H₂ gas are used to form a tungsten film. Note that an SiH₄ gas may be used instead of a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, for example, an In—Ga—ZnO film is formed using a deposition apparatus employing ALD, an In(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times to form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are used to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas are used to form a ZnO layer. Note that the order of these layers is not limited to this example. A mixed compound layer such as an In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by mixing these gases. Note that although an H₂O gas which is obtained by bubbling with an inert gas such as Ar may be used instead of an O₃ gas, it is preferable to use an O₃ gas, which does not contain H. Furthermore, instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gas may be used.

1-5. Structure Example 2 of Semiconductor Device

A structure example different from that of the transistor 100 in FIGS. 1A to 1C is described with reference to FIGS. 2A to 2C.

FIG. 2A is a top view of a transistor 150 that is a semiconductor device of one embodiment of the present invention. FIG. 2B is a cross-sectional view taken along dashed-dotted line X1-X2 illustrated in FIG. 2A, and FIG. 2C is a cross-sectional view taken along dashed-dotted line Y1-Y2 illustrated in FIG. 2A.

The transistor 150 differs from the transistor 100 in that opening portions 152 b and 152 c are provided in the channel width direction. The other portions of the transistor 150 are similar to those in the transistor 100 and have similar effects. The portions different from those in the transistor 100 are described below.

As illustrated in FIG. 2C, the oxide semiconductor film 120 b functioning as a second gate electrode is connected to the conductive film 104 functioning as a first gate electrode through opening portions 152 b and 152 c provided in the insulating films 106, 107, 114, and 116. Accordingly, the conductive film 104 and the oxide semiconductor film 120 b are supplied with the same potential.

Note that although the structure in which the opening portions 152 b and 152 c are provided so that the conductive film 104 and the oxide semiconductor film 120 b are connected to each other is described in this embodiment, one embodiment of the present invention is not limited thereto. For example, a structure in which only one of the opening portions 152 b and 152 c is provided so that the conductive film 104 and the oxide semiconductor film 120 b are connected to each other may be employed. Note that in the case where the conductive film 104 and the oxide semiconductor film 120 b are not connected to each other as in the transistor 100 shown in FIGS. 1A to 1C, it is possible to apply different potentials to the conductive film 104 and the oxide semiconductor film 120 b.

As illustrated in FIG. 2B, the oxide semiconductor film 108 is positioned to face each of the conductive film 104 functioning as a first gate electrode and the oxide semiconductor film 120 b functioning as a second gate electrode, and is sandwiched between the two films functioning as gate electrodes. The lengths in the channel length direction and the channel width direction of the oxide semiconductor film 120 b functioning as a second gate electrode are longer than those in the channel length direction and the channel width direction of the oxide semiconductor film 108. The whole oxide semiconductor film 108 is covered with the oxide semiconductor film 120 b with the insulating films 114 and 116 positioned therebetween. Since the oxide semiconductor film 120 b functioning as a second gate electrode is connected to the conductive film 104 functioning as a first gate electrode through the opening portion 152 b and 152 c provided in the insulating films 106, 107, 114, and 116, a side surface of the oxide semiconductor film 108 in the channel width direction faces the oxide semiconductor film 120 b functioning as a second gate electrode with the insulating films 114 and 116 positioned therebetween.

In other words, in the channel width direction of the transistor 150, the conductive film 104 functioning as a first gate electrode and the oxide semiconductor film 120 b functioning as a second gate electrode are connected to each other through the openings provided in the insulating films 106 and 107 functioning as first gate insulating films, and the insulating films 114 and 116 functioning as second gate insulating films; and the conductive film 104 and the oxide semiconductor film 120 b surround the oxide semiconductor film 108 with the insulating films 106 and 107 functioning as first gate insulating films, and the insulating films 114 and 116 functioning as second gate insulating films positioned therebetween.

Such a structure makes it possible that the oxide semiconductor film 108 included in the transistor 150 is electrically surrounded by electric fields of the conductive film 104 functioning as a first gate electrode and the oxide semiconductor film 120 b functioning as a second gate electrode. A device structure of a transistor, like that of the transistor 150, in which electric fields of a first gate electrode and a second gate electrode electrically surround an oxide semiconductor film where a channel region is formed can be referred to as a Surrounded channel (S-channel) structure.

Since the transistor 150 has the S-channel structure, an electric field for inducing a channel can be effectively applied to the oxide semiconductor film 108 by the conductive film 104 functioning as a first gate electrode; therefore, the current drive capability of the transistor 150 can be improved and high on-state current characteristics can be obtained. Since the on-state current can be increased, it is possible to reduce the size of the transistor 150. In addition, since the transistor 150 is surrounded by the conductive film 104 functioning as a first gate electrode and the oxide semiconductor film 120 b functioning as a second gate electrode, the mechanical strength of the transistor 150 can be increased.

<1-5-1. Effect of S-Channel Structure>

The effect obtained by employing the S-channel structure in a transistor including an oxide semiconductor (hereinafter referred to as OS-FET) and a transistor including low-temperature polysilicon (hereinafter referred to as LTPS-FET) is described below.

<1-5-2. Increase in Current Drive Capability (Field-Effect Mobility)>

First, the current density distribution in a thickness direction of an oxide semiconductor film (OS) in the case of employing the S-channel structure was calculated. FIG. 66A shows a structure used for the calculation and FIG. 66B shows the calculation results of the current density distribution.

As shown in FIG. 66A, the thickness of the oxide semiconductor film (OS) was set to 35 nm, and gate electrodes were located above and below the oxide semiconductor film in the structure used for the calculation. The distance between the lower gate electrode and the oxide semiconductor film was set to 250 nm, and the distance between the upper gate electrode and the oxide semiconductor film was set to 450 nm. The channel length of the oxide semiconductor film was set to 10 μm, and the length of each of a source electrode and a drain electrode in contact with the oxide semiconductor film was set to 1 μm. InGaZnO was used as the oxide semiconductor film. A voltage applied to the upper and lower gate electrodes (V_(g)) was set to 0.5 V.

FIG. 66B shows the current density distribution in the direction of the arrow A1-A2 in FIG. 66A, i.e., the thickness direction of the oxide semiconductor film. As shown in FIG. 66B, the current density distribution is approximately uniform in the thickness direction of the oxide semiconductor film. This is probably because the OS-FET is an accumulation-type FET with an intrinsic channel and an active layer is thin.

Next, an increase in the field-effect mobility of an OS-FET is described with reference to FIGS. 67A and 67B.

The field-effect mobility of an OS-FET was calculated with a device simulator. FIG. 67A shows a structure used for the calculation. As parameters used for the calculation, the channel length was set to 6 μm, the channel width was set to 50 μm, InGaZnO was used as an oxide semiconductor in a channel region, and the thickness of the oxide semiconductor was set to 35 nm. The thickness of a lower gate insulating film was set to 280 nm, the thickness of an upper gate insulating film was set to 480 nm, and the mobility of InGaZnO was set to 10 cm²/Vs.

The calculation was performed under two conditions. As Condition 1, an upper gate electrode and a lower gate electrode are not connected to each other, a voltage of 0 V was applied to the upper gate electrode, a voltage of 10 V was applied to the lower gate electrode, and a voltage of 10 V (V_(d)=10 V) was applied to a drain electrode. Note that Condition 1 is referred to as “Bottom Gate Driving” in some cases.

As Condition 2, the upper gate electrode and the lower gate electrode are connected to each other, a voltage of 10 V was applied to the upper gate electrode and the lower gate electrode, and a voltage of 10 V (V_(d)=10 V) was applied to the drain electrode. Note that Condition 2 is referred to as “Dual Gate Driving” in some cases.

FIG. 67B shows the calculation results of the field-effect mobility of the OS-FET under Condition 1 and Condition 2. In FIG. 67B, a solid line indicates a drain current (I_(d)), and a dashed line indicates field-effect mobility (μFE).

As shown in FIG. 67B, the field-effect mobility of the structure in which the upper gate electrode and the lower gate electrode are connected to each other (Dual Gate Driving) is approximately twice or more that of the structure in which the upper gate electrode and the lower gate electrode are not connected to each other (Bottom Gate Driving). Meanwhile, the mobility of the structure in which the upper gate electrode and the lower gate electrode are not connected to each other (Bottom Gate Driving) is lower than that of InGaZnO that is the parameter used for the calculation.

These results show that the field-effect mobility of the case of Dual Gate Driving using the OS-FET in which the upper and lower gate electrodes provided above and below the oxide semiconductor are connected to each other is increased as compared with the case of Bottom Gate Driving.

<1-5-3. Suppression of Characteristic Variation>

The variations in I_(d)-V_(g) characteristics of transistors with a Single Gate structure and an S-channel structure due to the variation in the donor density in a channel portion were calculated.

Note that the calculation conditions and the calculation model were V_(d)=10 V and an OS-FET, respectively.

The calculation results are shown in FIGS. 68A and 68B. Note that FIG. 68A shows the calculation results of the Single Gate structure, and FIG. 68B shows the calculation results of the S-channel structure.

As shown in FIGS. 68A and 68B, the variation in threshold voltage (V_(th)) and the amount of shift in the negative direction due to donor density in the S-channel structure are approximately half as much as those in the Single Gate structure. This suggests that, when the S-channel structure is used, the gate capacitance is increased and thus the variation in the threshold voltage (V_(th)) of the transistors is reduced.

Note that in the case where the V_(th) shift in the negative direction in the channel portion is represented as −ΔV_(th), −ΔV_(th) can be obtained using a charge amount Q_(D) due to a donor impurity and a gate capacitance Cox. Specifically, −ΔV_(th) can be expressed by the equation: −ΔV_(th)=−(Q_(D)/Cox).

When the S-channel structure is used, the gate capacitance is increased. Specifically, the gate capacitance is represented as the sum of the gate capacitance of the lower gate insulating film and the gate capacitance of the upper gate insulating film, i.e., Cox=Cox₁+Cox₂, where Cox₁ represents the gate capacitance of the upper gate insulating film and Cox₂ represents the gate capacitance of the lower gate insulating film. Therefore, by the increase in gate capacitance, the variation in threshold voltage (V_(th)) can be reduced even when the impurity density in the channel portion is varied.

Next, the case of an LTPS-FET is described. LTPS-FETs were manufactured, and V_(th) variations of the LTPS-FETs were compared to each other. FIGS. 69A and 69B and FIGS. 70A and 70B show evaluation results of the variations in I_(d)-V_(g) characteristics of the LTPS-FETs.

FIG. 69A shows the probability distribution of the threshold voltage (V_(th)) of an n-ch FET with a Single Gate structure in which an L length is 3 μm and a W length is 6 μm. FIG. 69B shows the probability distribution of the threshold voltage (V_(th)) of an n-ch FET with an S-channel structure in which an L length is 3 μm and a W length is 6 μm. FIG. 70A shows the probability distribution of the threshold voltage (V_(th)) of a p-ch FET with a Single Gate structure in which an L length is 3 μm and a W length is 6 μm. FIG. 70B shows the probability distribution of the threshold voltage (V_(th)) of a p-ch FET with an S-channel structure in which an L length is 3 μm and a W length is 6 μM.

As shown in FIGS. 69A and 69B and FIGS. 70A and 70B, the variation in the threshold voltage (V_(th)) of the transistor can be suppressed by employing the S-channel structure also in the LTPS-FET.

Next, the variation in the I_(d)-V_(g) characteristics in a subthreshold region in an LTPS-FET was calculated.

As the calculation conditions of the I_(d)-V_(g) characteristics in a subthreshold region, the channel length and the channel width of the transistor were set to 6 μm and 10 μm, respectively. Electron mobility and hole mobility of a crystal grain in LTPS were set to 300 cm²/Vs and 30 cm²/Vs, respectively, and the thickness of the LTPS was set to 50 nm. The electron mobility and the hole mobility of a crystal grain boundary in LTPS were set to 30 cm²/Vs and 3 cm²/Vs, respectively, the thickness of the LTPS was set to 50 nm, and the grain boundary width of the LTPS was set to 100 nm. The doping concentration of S/D regions was set to 5×10¹⁸ cm⁻³, and the doping concentration of a channel region was set to 1×10¹⁵ cm⁻³. The dielectric constant of upper and lower gate insulating films was set to 3.9, and the thickness of the upper and lower gate insulating films was set to 300 nm. The work function of upper and lower gate electrodes was set to 4.1 eV. The voltage applied to a drain electrode (V_(d)) was set to 3 V.

FIGS. 71A and 71B show calculation results of the variation in I_(d)-V_(g) characteristics in a subthreshold region. Note that FIG. 71A shows the calculation results of the Single Gate structure, and FIG. 71B shows the calculation results of the S-channel structure. Note that in FIGS. 71A and 71B, a structure in employed in which one crystal grain boundary of the LTPS is included in a channel. There are nine variations in the position of the crystal grain boundary, from the position in the vicinity of the end portion of a source electrode to the position in the vicinity of the end portion of the drain electrode. Thus, nine I_(d)-V_(g) characteristics are shown in each of FIGS. 71A and 71B.

As shown in FIGS. 71A and 71B, the shift of the threshold voltage (V_(th)) in the negative direction can be suppressed by employing the S-channel structure. However, the characteristic variation in the subthreshold region due to the crystal grain boundary is not changed largely even with the S-channel structure.

<1-5-4. Resistance to NBTS in S-Channel Structure>

Next, the resistance to negative bias temperature stress (NBTS) of an LTPS-FET and an OS-FET with an S-channel structure was calculated.

FIGS. 72A and 72B show structures used for the calculation. FIG. 72A shows the structure used for the calculation of an LTPS-FET, and FIG. 72B shows the structure used for the calculation of an OS-FET.

In FIG. 72A, the thickness of Poly-Si was set to 50 nm, the thickness of a gate electrode (Bottom Gate) located below the Poly-Si was set to 100 nm, and the thickness of a gate electrode (Top Gate) located above the Poly-Si was set to 100 nm. An insulating film was provided between the lower gate electrode (Bottom Gate) and the Poly-Si, and the thickness of the insulating film was set to 300 nm. An insulating film was provided between the upper gate electrode (Top Gate) and the Poly-Si, and the thickness of the insulating film was set to 300 nm.

In FIG. 72B, the thickness of OS was set to 50 nm, the thickness of a gate electrode (Bottom Gate) located below the OS was set to 100 nm, and the thickness of a gate electrode (Top Gate) located above the OS was set to 100 nm. An insulating film was provided between the lower gate electrode (Bottom Gate) and the OS, and the thickness of the insulating film was set to 300 nm. An insulating film was provided between the upper gate electrode (Top Gate) and the OS, and the thickness of the insulating film was set to 300 nm.

In setting the calculation conditions, NBTS was assumed where −10 V was applied to each of the upper gate electrode (Top Gate) and the lower gate electrode (Bottom Gate).

FIGS. 73A and 73B show the calculation results of the potential distribution in the thickness direction when the NBTS was applied. Note that FIG. 73A shows the calculation results of the LTPS-EFT, and FIG. 73B shows the calculation results of the OS-FET.

In the LTPS-FET, as shown in FIG. 73A, a potential between the upper gate electrode (Top Gate) and the lower gate electrode (Bottom Gate) is changed. That is, an electric field is applied to the Poly-Si, and a stress due to the electric field is applied to the Poly-Si even when a voltage applied to the upper gate electrode (Top Gate) and the lower gate electrode (Bottom Gate) is off. It is suggested that this is due to the generation of minority carriers by inversion in the LTPS-FET.

In the OS-FET, however, as shown in FIG. 73B, a potential between the upper gate electrode (Top Gate) and the lower gate electrode (Bottom Gate) is not changed or the potential is approximately uniform. This suggests that the upper gate electrode (Top Gate) and the lower gate electrode (Bottom Gate) have the same potential and an electric field is not applied to the OS and the insulating films. That is, it is suggested that the resistance to the NBTS is significantly increased by employing the S-channel structure in the OS-FET.

<1-5-5. Length in W Length Direction in S-Channel Structure>

The length in a W length direction in an S-channel structure is described. Here, transistors shown in FIGS. 74A to 74C and FIGS. 75A to 75C were manufactured and positive bias temperature stress (PBTS) tests were performed on the transistors.

FIG. 74A is a top view of a transistor 400, FIG. 74B is a cross-sectional view taken along the dashed-dotted line X1-X2 shown in FIG. 74A, and FIG. 74C is a cross-sectional view taken along the dashed-dotted line Y1-Y2 shown in FIG. 74A.

FIG. 75A is a top view of a transistor 400A, FIG. 75B is a cross-sectional view taken along the dashed-dotted line X1-X2 shown in FIG. 75A, and FIG. 75C is a cross-sectional view taken along the dashed-dotted line Y1-Y2 shown in FIG. 75A.

The transistor 400 and the transistor 400A each include a conductive film 404 over a substrate 402, insulating films 406 and 407 over the conductive film 404, an oxide semiconductor film 408 over the insulating film 407, a conductive film 412 a electrically connected to the oxide semiconductor film 408, a conductive film 412 b electrically connected to the oxide semiconductor film 408, insulating films 414 and 416 over the oxide semiconductor film 408, the conductive film 412 a, and the conductive film 412 b, a conductive film 420 over the insulating film 416, and an insulating film 418 over the insulating film 416 and the conductive film 420.

Note that in the transistors 400 and 400A, the oxide semiconductor film 408 has a stacked-layer structure of an oxide semiconductor film 408 a and an oxide semiconductor film 408 b over the oxide semiconductor film 408 a. In the transistors 400 and 400A, the conductive film 404 functions as a first gate electrode, and the conductive film 420 functions as a second gate electrode. Furthermore, in the transistors 400 and 400A, the insulating films 406 and 407 function as a first gate insulating film, and the insulating film 414 and 416 function as a second gate insulating film. In the transistors 400 and 400A, the conductive film 412 a functions as a source electrode, and the conductive film 412 b functions as a drain electrode.

Note that the transistor 400 shown in FIGS. 74A to 74C and the transistor 400A shown in FIGS. 75A to 75C differ in the length of the conductive film 420 in the channel width W direction, and components of the transistors 400 and 400A except the conductive film 420 are the same.

Specifically, in the transistor 400 shown in FIGS. 74A to 74C, the conductive film 420 functioning as a second gate electrode is shorter than the oxide semiconductor film 408 in the W length direction. Meanwhile, in the transistor 400A shown in FIGS. 75A to 75C, the conductive film 420 functioning as a second gate electrode in the W length direction is longer than the oxide semiconductor film 408 in the W length direction. In other words, the transistor 400A has a structure in which a side surface of the oxide semiconductor film 408 in the W length direction is covered with the conductive film 420 functioning as a second gate electrode, i.e., an S-channel structure.

Note that structures of the conductive film, the insulating film, and the oxide semiconductor film of each of the transistor 400 and the transistor 400A are as follows.

A 200-nm-thick tungsten film was formed as the conductive film 404, and a 100-nm-thick ITSO film was formed as the conductive film 420. The conductive films 412 a and 412 b each had a stacked-layer structure of a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a 200-nm-thick titanium film. A 35-nm-thick IGZO film (In:Ga:Zn=3:1:2 [atomic ratio]) was formed as the oxide semiconductor film 408 a, and a 15-nm-thick IGZO film (In:Ga:Zn=1:3:2 [atomic ratio]) was formed as the oxide semiconductor film 408 b. A 400-nm-thick silicon nitride film was formed as the insulating film 406, and a 50-nm-thick silicon oxynitride film was formed as the insulating film 407. A 50-nm-thick silicon oxynitride film was formed as the insulating film 414, and a 400-nm-thick silicon oxynitride film was formed as the insulating film 416. A 100-nm-thick silicon nitride film was formed as the insulating film 418.

FIG. 76A shows I_(d)-V_(g) characteristics of a transistor corresponding to the transistor 400 shown in FIGS. 74A to 74C before and after the PBTS test. FIG. 76B shows I_(d)-V_(g) characteristics of a transistor corresponding to the transistor 400A shown in FIGS. 75A to 75C before and after the PBTS test.

Note that in FIGS. 76A and 76B, the first vertical axis indicates I_(d), the second vertical axis indicates μFE, and the horizontal axis indicates V_(g). In FIGS. 76A and 76B, a solid line indicates the results before the stress test, and a dashed line indicates the results after the stress test.

As shown in FIG. 76A, in the structure where the side surface of the oxide semiconductor film 408 is not covered with the conductive film 420 functioning as a second gate electrode, the I_(d)-V_(g) characteristics are degraded after the PBTS test. As shown in FIG. 76B, in the structure where the side surface of the oxide semiconductor film 408 is covered with the conductive film 420 functioning as a second gate electrode, i.e., in the S-channel structure, a change in the I_(d)-V_(g) characteristics is not observed or is extremely small after the PBTS test.

It is suggested that the degradation of the I_(d)-V_(g) characteristics shown in FIGS. 76A and 76B occurred because the side end portion of the island-like oxide semiconductor film 408 becomes n-type and a parasitic channel is formed in the transistor 400.

<1-5-6. V_(d) Dependence of I_(d)-V_(g) Characteristics>

A transistor corresponding to the transistor 400A shown in FIGS. 75A to 75C (a transistor with an S-channel structure) and a transistor without the conductive film 420 functioning as a second gate electrode included in the transistor 400A (a transistor with a Single Gate structure) were manufactured, and the V_(d) dependence of the I_(d)-V_(g) characteristics of the manufactured transistors were evaluated.

Structures of an insulating film, a conductive film, and an oxide semiconductor film in each of the manufactured transistors were the same as those described in <1-5-5. Length in W length direction in S-channel structure>. The transistors each had a channel length L of 2 μm and a channel width W of 50 μm.

The I_(d)-V_(g) characteristics of the manufactured transistors were evaluated. Note that two conditions, i.e., V_(d)=0.1 V and V_(d)=10 V, were employed in the I_(d)-V_(g) characteristics measurement.

The measurement results of the I_(d)-V_(g) characteristics of the transistors are shown in FIGS. 77A and 77B. FIG. 77A shows the results of the transistor with the Single Gate structure. FIG. 77B shows the results of the transistor with the S-channel structure.

In the case of the Single Gate structure, different rising characteristics of the threshold voltages are exhibited because of different V_(d) voltages, as shown in FIG. 77A. Specifically, the threshold voltage is shifted in the negative direction in the case of the Single Gate structure. The negative shift of the threshold voltage is significant particularly when a high drain voltage (V_(d)=10 V) is applied.

It is suggested that the above-described phenomenon is caused by a drain-induced barrier lowering (DIBL) effect. The DIBL effect is a phenomenon in which the band barrier between a source and a channel is lowered by a drain electric field.

In contrast, in the case of the S-channel structure, the rising characteristics of the threshold voltages are approximately the same even with different V_(d) voltages, as shown in FIG. 77B. It is suggested that this is because the drain electric field is shielded by the upper and lower gate electrodes.

Next, the V_(d) dependence of I_(d)-V_(g) characteristics of LTPS-FETs was evaluated. As the LTPS-FETs, an nch transistor with L/W=6/50 μm and a pch transistor with L/W=6/50 μm were manufactured. As the transistor structures, the above-described Single Gate structure and S-channel structure were used.

The measurement results of the I_(d)-V_(g) characteristics of the LTPS-FETs are shown in FIGS. 78A and 78B and FIGS. 79A and 79B. FIG. 78A shows the results of the I_(d)-V_(g) characteristics of the nch transistor with the Single Gate structure. FIG. 78B shows the results of the I_(d)-V_(g) characteristics of the nch transistor with the S-channel structure. FIG. 79A shows the results of the I_(d)-V_(g) characteristics of the pch transistor with the Single Gate structure. FIG. 79B shows the results of the I_(d)-V_(g) characteristics of the pch transistor with the S-channel structure.

As shown in FIGS. 78A and 78B and FIGS. 79A and 79B, using the S-channel structure instead of the Single Gate structure can suppress the variation in the rising characteristics of the threshold voltages with different V_(d) in the LTPS-FETs as in the OS-FETs. That is, also in the LTPS-FET, the DIBL effect can be suppressed by the S-channel structure.

Furthermore, as shown in FIGS. 78A and 78B and FIGS. 79A and 79B, using the S-channel structure can also achieve higher field-effect mobility than using the Single Gate structure in the LTPS-FETs as in the OS-FETs. The field-effect mobility of the LTPS-FET with the S-channel structure is approximately 1.6 times that of the LTPS-FET with the Single Gate structure.

<1-5-7. Saturation Characteristics of I_(d)-V_(d) Characteristics>

The saturation characteristics in the I_(d)-V_(d) characteristics of transistors with a Single Gate structure and an S-channel structure are described.

A transistor corresponding to the transistor 400A shown in FIGS. 75A to 75C (a transistor with an S-channel structure) and a transistor without the conductive film 420 functioning as a second gate electrode included in the transistor 400A (a transistor with a Single Gate structure) were manufactured, and the saturation characteristics in the I_(d)-V_(d) characteristics of the manufactured transistors were evaluated.

Structures of an insulating film, a conductive film, and an oxide semiconductor film in each of the manufactured transistors were the same as those described in <1-5-5. Length in W length direction in S-channel structure>. The transistors each had a channel length L of 3 μm and a channel width W of 50 μm.

FIGS. 80A and 80B show the I_(d)-V_(d) characteristics of the transistors with the Single Gate structure and the S-channel structure. FIG. 80A shows the results of the I_(d)-V_(d) characteristics of the transistor with the Single Gate structure. FIG. 80B shows the results of the I_(d)-V_(d) characteristics of the transistor with the S-channel structure.

As shown in FIGS. 80A and 80B, the saturation characteristics of the I_(d)-V_(d) characteristics are improved in the case of the S-channel structure. This is probably because the aforementioned DIBL effect is suppressed. In the OS-FET with the Single Gate structure, drain current (I_(d)) is increased by the DIBL effect also in a saturation region of the I_(d)-V_(d) characteristics when the drain voltage is high.

The FET exhibiting improved saturation characteristics in the I_(d)-V_(d) characteristics can be suitably used as an FET for driving a display device including an organic EL element, for example.

Next, the saturation characteristics of the I_(d)-V_(d) characteristics of LTPS-FETs is described with reference to FIGS. 81A to 81C and FIGS. 82A to 82C.

The I_(d)-V_(d) characteristics of LTPS-FETs are shown in FIGS. 81A to 81C and FIGS. 82A to 82C. FIG. 81A shows the measurement results of the I_(d)-V_(d) characteristics of a transistor with a Single Gate structure with L/W=6/3 μm. FIG. 81B shows the measurement results of the I_(d)-V_(d) characteristics of a transistor with a Single Gate structure with L/W=10/3 μm. FIG. 81C shows the measurement results of the I_(d)-V_(d) characteristics of a transistor with a Single Gate structure with L/W=50/3 μm. FIG. 82A shows the measurement results of the I_(d)-V_(d) characteristics of a transistor with an S-channel structure with L/W=6/3 μm. FIG. 82B shows the measurement results of the I_(d)-V_(d) characteristics of a transistor with an S-channel structure with L/W=10/3 μm. FIG. 82C shows the measurement results of the I_(d)-V_(d) characteristics of a transistor with an S-channel structure with L/W=50/3 μm.

As shown in FIGS. 81A to 81C and FIGS. 82A to 82C, using the S-channel structure can suppress the DIBL effect and improves the saturation characteristics of the I_(d)-V_(d) characteristics also in the LTPS-FETs. However, a phenomenon in which the drain current is increased again by the increase in the absolute value of drain voltage (V_(d)) is observed, unlike the above-described OS-FET. In particular, in the transistor with short L, the phenomenon in which the drain current is increased again is clearly observed.

The above-described phenomenon is thought to be caused by avalanche breakdown (impact ionization) occurred at an end portion of a drain due to hot carriers (holes) generated by the increase in drain electric field. A possible reason for this is a smaller bandgap of silicon used as LTPS than that of an oxide semiconductor film; because of its smaller bandgap, silicon is affected more largely by the impact ionization than an oxide semiconductor film.

<1-5-8. Subthreshold Swing>

Subthreshold swings of a transistor with a Single Gate structure and a transistor with an S-channel structure were calculated. Note that the term “subthreshold swing” is a gate voltage necessary for increasing a current (subthreshold current) between a source electrode and a drain electrode by one digit, and the smaller a subthreshold swing is, the steeper the slope of the subthreshold current with respect to the gate voltage is and the more excellent the switching characteristics are.

FIG. 83 shows the calculation results of the I_(d)-V_(g) characteristics of the transistor with the Single Gate structure and the transistor with the S-channel structure. In FIG. 83, the dashed line shows the calculation results of the I_(d)-V_(g) characteristics of the transistor with the Single Gate structure, and the solid line shows the calculation results of the I_(d)-V_(g) characteristics of the transistor with the S-channel structure.

For the calculation of the subthreshold swings, OS-FETs with L/W=2/50 μm were assumed.

As shown in FIG. 83, the subthreshold swing is decreased in the case of the S-channel structure. This is probably because the controllability of a channel is improved by upper and lower gate electric fields in the case of the S-channel structure.

The effects achieved by using the S-channel structure in the OS-EFT and the LTPS-FET are summarized in Table 1.

TABLE 1 OS-FET LTPS-FET Resistance to −GBT stress ⊚ X Resistance to +GBT stress ◯ —*¹⁾ Improvement in current ◯ ◯ drive capability (mobility) Improvement in ◯ ◯ subthreshold swing Effect of suppressing ◯ Δ characteristics variation Effect of suppressing ⊚ ◯ DIBL Saturation characteristics ◯ X of I_(d)-V_(d) characteristics Note: *¹⁾— means “Not determined”.

As shown in Table 1, although several effects are expected in the LTPS-FET with the S-channel structure, more effects are achieved in the OS-FET with the S-channel structure.

1-6. Structure Example 3 of Semiconductor Device

A structure example different from that of the transistor 100 in FIGS. 1A to 1C is described with reference to FIGS. 3A to 3C.

FIG. 3A is a top view of a transistor 160 that is a semiconductor device of one embodiment of the present invention. FIG. 3B is a cross-sectional view taken along dashed dotted line X1-X2 in FIG. 3A, and FIG. 3C is a cross-sectional view taken along dashed dotted line Y1-Y2 in FIG. 3A.

The transistor 160 differs from the transistor 100 in the structures of the oxide semiconductor film 120 a functioning as a pixel electrode and the oxide semiconductor film 120 b functioning as a second gate electrode. The other portions of the transistor 160 are similar to those in the transistor 100 and have similar effects. The portions different from those in the transistor 100 are described below.

The oxide semiconductor film 120 a functioning as a pixel electrode in the transistor 160 has a stacked-layer structure of an oxide semiconductor film 120 a_1 and an oxide semiconductor film 120 a_2. The oxide semiconductor film 120 b functioning as a second gate electrode in the transistor 160 has a stacked-layer structure of an oxide semiconductor film 120 b_1 and an oxide semiconductor film 120 b_2.

When the oxide semiconductor film 120 a and the oxide semiconductor film 120 b each have a stacked-layer structure, oxygen can be suitably introduced into the insulating film 116. Moreover, when the oxide semiconductor film 120 a and the oxide semiconductor film 120 b each have a stacked-layer structure, entry of hydrogen contained in the insulating film 118 into the insulating film 116 can be suppressed.

The atomic ratio between metal elements in a sputtering target used for forming the oxide semiconductor film 120 a_1 and the oxide semiconductor film 120 b_1 preferably satisfies In≦M. The atomic ratio between metal elements in such a sputtering target is In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:6, or the like.

The atomic ratio between metal elements in a sputtering target used for forming the oxide semiconductor film 120 a_2 and the oxide semiconductor film 120 b_2 preferably satisfies In≧M. The atomic ratio between metal elements in such a sputtering target is In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=2:1:3, In:M:Zn=3:1:2, In:M:Zn=4:2:4.1, or the like.

In the case of forming the oxide semiconductor film 120 a and the oxide semiconductor film 120 b each having a two-layer structure, the flow rate of an oxygen gas for forming a lower oxide semiconductor film is preferably set higher than the flow rate of an oxygen gas for forming an upper oxide semiconductor film. When the lower oxide semiconductor film is formed using an oxygen gas with a higher flow rate, oxygen can be suitably added to the insulating film 116. When the upper oxide semiconductor film is formed using an oxygen gas with a lower flow rate, the resistance of the oxide semiconductor film can be reduced.

A semiconductor device including the oxide semiconductor film 120 a and the oxide semiconductor film 120 b each having the above-described structure can have higher reliability.

1-7. Structure Example 4 of Semiconductor Device

A structure example different from that of the transistor 150 in FIGS. 2A to 2C is described with reference to FIGS. 4A to 4C.

FIG. 4A is a top view of a transistor 170 that is a semiconductor device of one embodiment of the present invention. FIG. 4B is a cross-sectional view taken along dashed-dotted line X1-X2 illustrated in FIG. 4A, and FIG. 4C is a cross-sectional view taken along dashed-dotted line Y1-Y2 illustrated in FIG. 4A.

The transistor 170 differs from the transistor 150 in the structures of the oxide semiconductor film 120 a functioning as a pixel electrode and the oxide semiconductor film 120 b functioning as a second gate electrode. The other portions of the transistor 170 are similar to those in the transistor 150 and have similar effects. The structures of the oxide semiconductor film 120 a and the oxide semiconductor film 120 b may be similar to those of the transistor 160 described above.

1-8. Structure Example 5 of Semiconductor Device

Modification examples of the transistor 150 shown in FIGS. 2A to 2C and a modification example of the transistor 170 shown in FIGS. 4A to 4C are described with reference to FIGS. 5A to 5D, FIGS. 6A and 6B, and FIGS. 7A to 7D.

FIGS. 5A and 5B are cross-sectional views of a modification example of the transistor 150 in FIGS. 2B and 2C. FIGS. 5C and 5D are cross-sectional views of a modification example of the transistor 170 in FIGS. 4B and 4C.

A transistor 150A in FIGS. 5A and 5B has the same structure as the transistor 150 in FIGS. 2B and 2C except that the oxide semiconductor film 108 has a three-layer structure. Specifically, the oxide semiconductor film 108 of the transistor 150A includes an oxide semiconductor film 108 a, the oxide semiconductor film 108 b, and the oxide semiconductor film 108 c.

A transistor 170A in FIGS. 5C and 5D has the same structure as the transistor 170 in FIGS. 4B and 4C except that the oxide semiconductor film 108 has a three-layer structure. Specifically, the oxide semiconductor film 108 of the transistor 170A includes the oxide semiconductor film 108 a, the oxide semiconductor film 108 b, and the oxide semiconductor film 108 c.

Here, band structures of insulating films in contact with the oxide semiconductor films 108 a, 108 b, and 108 c and insulating films in contact with the oxide semiconductor films 108 b and 108 c are described with reference to FIGS. 6A and 6B.

FIG. 6A shows an example of a band structure in the thickness direction of a stack including the insulating film 107, the oxide semiconductor films 108 a, 108 b, and 108 c, and the insulating film 114. FIG. 6B shows an example of a band structure in the thickness direction of a stack including the insulating film 107, the oxide semiconductor films 108 b and 108 c, and the insulating film 114. For easy understanding, the conduction band minimum (Ec) of each of the insulating film 107, the oxide semiconductor films 108 a, 108 b, and 108 c, and the insulating film 114 is shown in the band diagrams.

In the band structure of FIG. 6A, a silicon oxide film is used as each of the insulating films 107 and 114, an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:1:1.2 is used as the oxide semiconductor film 108 a, an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=4:2:4.1 is used as the oxide semiconductor film 108 b, and an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:1:1.2 is used as the oxide semiconductor film 108 c.

In the band structure of FIG. 6B, a silicon oxide film is used as each of the insulating films 107 and 114, an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=4:2:4.1 is used as the oxide semiconductor film 108 b, and an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:1:1.2 is used as the oxide semiconductor film 108 c.

As illustrated in FIGS. 6A and 6B, the energy level of the conduction band minimum gradually varies between the oxide semiconductor film 108 a and the oxide semiconductor film 108 b and between the oxide semiconductor film 108 b and the oxide semiconductor film 108 c. In other words, the energy level at the bottom of the conduction band is continuously varied or continuously connected. To obtain such a band structure, there exists no impurity, which forms a defect state such as a trap center or a recombination center, at the interface between the oxide semiconductor film 108 a and the oxide semiconductor film 108 b or at the interface between the oxide semiconductor film 108 b and the oxide semiconductor film 108 c.

To form a continuous junction between the oxide semiconductor film 108 a and the oxide semiconductor film 108 b and between the oxide semiconductor film 108 b and the oxide semiconductor film 108 c, it is necessary to form the films successively without exposure to the air by using a multi-chamber deposition apparatus (sputtering apparatus) provided with a load lock chamber.

With the band structure of FIG. 6A or FIG. 6B, the oxide semiconductor film 108 b serves as a well, and a channel region is formed in the oxide semiconductor film 108 b in the transistor with the stacked-layer structure.

By providing the oxide semiconductor film 108 a and/or the oxide semiconductor film 108 c, the oxide semiconductor film 108 b is distanced from trap states that could be formed in the vicinity of the interface between the oxide semiconductor film 108 b and the insulating film (the insulating film 107 or the insulating film 114).

In addition, in some cases, the trap states are more distant from the vacuum level than the energy level of the conduction band minimum (Ec) of the oxide semiconductor film 108 b functioning as a channel region, so that electrons are likely to be accumulated in the trap states. When the electrons are accumulated in the trap states, the electrons become negative fixed electric charge, so that the threshold voltage of the transistor is shifted in the positive direction. Therefore, it is preferable that the trap states be closer to the vacuum level than the energy level of the conduction band minimum (Ec) of the oxide semiconductor film 108 b. Such a structure inhibits accumulation of electrons in the trap states. As a result, the on-state current and the field-effect mobility of the transistor can be increased.

The energy level of the conduction band minimum of each of the oxide semiconductor films 108 a and 108 c is closer to the vacuum level than that of the oxide semiconductor film 108 b. A typical difference between the energy level of the conduction band minimum of the oxide semiconductor film 108 b and the energy level of the conduction band minimum of each of the oxide semiconductor films 108 a and 108 c is 0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eV or less. That is, the difference between the electron affinity of each of the oxide semiconductor films 108 a and 108 c and the electron affinity of the oxide semiconductor film 108 b is 0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eV or less.

In such a structure, the oxide semiconductor film 108 b serves as a main path of current. That is, the oxide semiconductor film 108 b functions as a channel region, and the oxide semiconductor films 108 a and 108 c function as oxide insulating films. In addition, since the oxide semiconductor films 108 a and 108 c each include one or more metal elements included in the oxide semiconductor film 108 b in which a channel region is formed, interface scattering is less likely to occur at the interface between the oxide semiconductor film 108 a and the oxide semiconductor film 108 b or at the interface between the oxide semiconductor film 108 b and the oxide semiconductor film 108 c. Thus, the transistor can have high field-effect mobility because the movement of carriers is not hindered at the interface.

To prevent each of the oxide semiconductor films 108 a and 108 c from functioning as part of a channel region, a material having sufficiently low conductivity is used for the oxide semiconductor films 108 a and 108 c. Thus, each of the oxide semiconductor films 108 a and 108 c can also be referred to as “oxide insulating film” owing to its physical property and/or function. Alternatively, a material which has a smaller electron affinity (a difference in energy level between the vacuum level and the conduction band minimum) than the oxide semiconductor film 108 b and has a difference in energy level in the conduction band minimum from the oxide semiconductor film 108 b (band offset) is used for the oxide semiconductor films 108 a and 108 c. Furthermore, to inhibit generation of a difference between threshold voltages due to the value of the drain voltage, it is preferable to form the oxide semiconductor films 108 a and 108 c using a material whose energy level of the conduction band minimum is closer to the vacuum level than the energy level of the conduction band minimum of the oxide semiconductor film 108 b. For example, a difference in energy level between the conduction band minimum of the oxide semiconductor film 108 b and the conduction band minimum of the oxide semiconductor films 108 a and 108 c is 0.2 eV or more, preferably 0.5 eV or more.

It is preferable that the oxide semiconductor films 108 a and 108 c not have a spinel crystal structure. This is because if the oxide semiconductor films 108 a and 108 c have a spinel crystal structure, constituent elements of the conductive films 112 a and 112 b might be diffused to the oxide semiconductor film 108 b at the interface between the spinel crystal structure and another region. Note that each of the oxide semiconductor film 108 a and 108 c is preferably a CAAC-OS, which is described later, in which case a higher blocking property against constituent elements of the conductive films 112 a and 112 b, for example, copper elements, is obtained.

The thickness of each of the oxide semiconductor films 108 a and 108 c is greater than or equal to a thickness that is capable of inhibiting diffusion of the constituent elements of the conductive films 112 a and 112 b to the oxide semiconductor film 108 b, and less than a thickness that inhibits supply of oxygen from the insulating film 114 to the oxide semiconductor film 108 b. For example, when the thickness of each of the oxide semiconductor films 108 a and 108 c is greater than or equal to 10 nm, diffusion of the constituent elements of the conductive films 112 a and 112 b to the oxide semiconductor film 108 b can be inhibited. When the thickness of each of the oxide semiconductor films 108 a and 108 c is less than or equal to 100 nm, oxygen can be effectively supplied from the insulating film 114 to the oxide semiconductor film 108 b.

Although the example where an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements, In:Ga:Zn=1:1:1.2, is used as each of the oxide semiconductor films 108 a and 108 c is described in this embodiment, one embodiment of the present invention is not limited thereto. For example, an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements, In:Ga:Zn=1:1:1, In:Ga:Zn=1:3:2, In:Ga:Zn=1:3:4, or In:Ga:Zn=1:3:6, may be used as each of the oxide semiconductor films 108 a and 108 c.

When the oxide semiconductor films 108 a and 108 c are formed using a metal oxide target having an atomic ratio of In:Ga:Zn=1:1:1, the oxide semiconductor films 108 a and 108 c have an atomic ratio of In:Ga:Zn=1:β1 (0<β1≦2):β2 (0<β2≦2) in some cases. When the oxide semiconductor films 108 a and 108 c are formed using a metal oxide target having an atomic ratio of In:Ga:Zn=1:3:4, the oxide semiconductor films 108 a and 108 c have an atomic ratio of In:Ga:Zn=1:β3 (1≦β3≦5):β4 (2≦β4≦6) in some cases. When the oxide semiconductor films 108 a and 108 c are formed using a metal oxide target having an atomic ratio of In:Ga:Zn=1:3:6, the oxide semiconductor films 108 a and 108 c have an atomic ratio of In:Ga:Zn=1:β5 (1≦β5≦5):β6 (4≦β6≦8) in some cases.

The drawings illustrate an example where the oxide semiconductor film 108 c in the transistor 150 and the oxide semiconductor film 108 c in the transistor 150A have a small thickness in a region which is not covered with the conductive films 112 a and 112 b, that is, an example where part of the oxide semiconductor film has a depressed portion. However, one embodiment of the present invention is not limited thereto, and the oxide semiconductor film does not necessarily have a depressed region in a region which is not covered with the conductive films 112 a and 112 b. FIGS. 7A to 7D illustrate examples in this case. FIGS. 7A to 7D are cross-sectional views illustrating examples of the semiconductor device. FIGS. 7A and 7B illustrate a structure where the oxide semiconductor film 108 c in the transistor 150 does not have a depressed portion, and FIGS. 7C and 7D illustrate a structure where the oxide semiconductor film 108 c in the transistor 150A does not have a depressed portion.

The structures of the transistors of this embodiment can be freely combined with each other.

1-9. Method for Manufacturing Semiconductor Device

Next, a method for manufacturing the transistor 100 that is a semiconductor device of one embodiment of the present invention is described with reference to FIGS. 8A to 8F, FIGS. 9A to 9F, FIGS. 10A to 10F, and FIGS. 11A to 11F.

FIGS. 8A to 8F, FIGS. 9A to 9F, FIGS. 10A to 10F, and FIGS. 11A to 11F are cross-sectional views showing a method for manufacturing a semiconductor device. Cross-sectional views in the channel length direction are shown in FIGS. 8A, 8C, and 8E, FIGS. 9A, 9C, and 9E, FIGS. 10A, 10C, and 10E, and FIGS. 11A, 11C, and 11E. Cross-sectional views in the channel width direction are shown in FIGS. 8B, 8D, and 8F, FIGS. 9B, 9D, and 9F, FIGS. 10B, 10D, and 10F, and FIGS. 11B, 11D, and 11F.

First, a conductive film is formed over the substrate 102 and processed through a lithography process and an etching process, whereby the conductive film 104 functioning as a first gate electrode is formed. Then, the insulating films 106 and 107 which function as a first gate insulating film are formed over the conductive film 104 (see FIGS. 8A and 8B).

In this embodiment, a glass substrate is used as the substrate 102, and as the conductive film 104 functioning as a first gate electrode, a 100-nm-thick tungsten film is formed by a sputtering method. As the insulating film 106, a 400-nm-thick silicon nitride film is formed by a PECVD method. As the insulating film 107, a 50-nm-thick silicon oxynitride film is formed by a PECVD method.

Note that the insulating film 106 can have a stacked-layer structure of silicon nitride films. Specifically, the insulating film 106 can have a three-layer structure of a first silicon nitride film, a second silicon nitride film, and a third silicon nitride film. An example of the three-layer structure is as follows.

For example, the first silicon nitride film can be formed to have a thickness of 50 nm under the conditions where silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccm are supplied as a source gas to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and the power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

The second silicon nitride film can be formed to have a thickness of 300 nm under the conditions where silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 2000 sccm are supplied as a source gas to the reaction chamber of the PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and the power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

The third silicon nitride film can be formed to have a thickness of 50 nm under the conditions where silane at a flow rate of 200 sccm and nitrogen at a flow rate of 5000 sccm are supplied as a source gas to the reaction chamber of the PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and the power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

Note that the first silicon nitride film, the second silicon nitride film, and the third silicon nitride film can each be formed at a substrate temperature of 350° C. or lower.

When the insulating film 106 has the three-layer structure of silicon nitride films, for example, in the case where a conductive film including Cu is used as the conductive film 104, the following effect can be obtained.

The first silicon nitride film can inhibit diffusion of a copper (Cu) element from the conductive film 104. The second silicon nitride film has a function of releasing hydrogen and can improve withstand voltage of the insulating film functioning as a gate insulating film. The third silicon nitride film releases a small amount of hydrogen and can inhibit diffusion of hydrogen released from the second silicon nitride film.

The insulating film 107 is preferably an insulating film including oxygen to improve characteristics of an interface with the oxide semiconductor film 108 (specifically the oxide semiconductor film 108 b) formed later.

Next, an oxide semiconductor film 108 b_0 and an oxide semiconductor film 108 c_0 are formed over the insulating film 107 (see FIGS. 8C, 8D, 8E, and 8F).

FIGS. 8C and 8D are schematic cross-sectional views showing an inner portion of a deposition apparatus when the oxide semiconductor film 108 b_0 is formed over the insulating film 107. In FIGS. 8C and 8D, a sputtering apparatus is used as the deposition apparatus, and a target 191 placed inside the sputtering apparatus and plasma 192 formed under the target 191 are schematically shown.

When the oxide semiconductor film 108 b_0 is formed, plasma discharge is performed in an atmosphere containing a first oxygen gas. At this time, oxygen is added to the insulating film 107 over which the oxide semiconductor film 108 b_0 is to be formed. When the oxide semiconductor film 108 b_0 is formed, an inert gas (e.g., a helium gas, an argon gas, or a xenon gas) and the first oxygen gas may be mixed.

The first oxygen gas is mixed at least when the oxide semiconductor film 108 b_0 is formed. The proportion of the first oxygen gas in a deposition gas for forming the oxide semiconductor film 108 b_0 is higher than 0% and lower than or equal to 100%, preferably higher than or equal to 10% and lower than or equal to 100%, more preferably higher than or equal to 30% and lower than or equal to 100%.

In FIGS. 8C and 8D, oxygen or excess oxygen added to the insulating film 107 is schematically shown by arrows of broken lines.

The oxide semiconductor films 108 b_0 and 108 c_0 may be formed at the same substrate temperature or different substrate temperatures. Note that the oxide semiconductor films 108 b_0 and 108 c_0 are preferably formed at the same substrate temperature, in which case the manufacturing cost can reduced.

The oxide semiconductor film 108 is formed at a substrate temperature higher than or equal to room temperature and lower than 340° C., preferably higher than or equal to room temperature and lower than or equal to 300° C., further preferably higher than or equal to 100° C. and lower than or equal to 250° C., still further preferably higher than or equal to 100° C. and lower than or equal to 200° C., for example. The oxide semiconductor film 108 is formed while being heated, so that the crystallinity of the oxide semiconductor film 108 can be increased. On the other hand, in the case where a large-sized glass substrate (e.g., the 6th generation to the 10th generation) is used as the substrate 102 and the oxide semiconductor film 108 is formed at a substrate temperature higher than or equal to 150° C. and lower than 340° C., the substrate 102 might be changed in shape (distorted or warped). In the case where a large-sized glass substrate is used, the change in the shape of the glass substrate can be suppressed by forming the oxide semiconductor film 108 at a substrate temperature higher than or equal to 100° C. and lower than 150° C.

In addition, increasing the purity of a sputtering gas is necessary. For example, as an oxygen gas or an argon gas used for a sputtering gas, a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, still further preferably −120° C. or lower is used, whereby entry of moisture or the like into the oxide semiconductor film can be minimized.

When the oxide semiconductor film is formed by a sputtering method, each chamber of a sputtering apparatus is preferably evacuated to a high vacuum (to the degree of approximately 5×10⁻⁷ Pa to 1×10⁻⁴ Pa, for example) by an adsorption vacuum pump such as a cryopump so that water and the like acting as impurities for the oxide semiconductor film are removed as much as possible. Alternatively, a turbo molecular pump and a cold trap are preferably combined so as to prevent a backflow of a gas, especially a gas containing carbon or hydrogen from an exhaust system to the inside of the chamber.

After the oxide semiconductor film 108 b_0 is formed, the oxide semiconductor film 108 c_0 is successively formed over the oxide semiconductor film 108 b_0. Note that when the oxide semiconductor film 108 c_0 is formed, plasma discharge is performed in an atmosphere containing a second oxygen gas.

Note that the proportion of the first oxygen gas for forming the oxide semiconductor film 108 b_0 may be the same as or different from the proportion of the second oxygen gas for forming the oxide semiconductor film 108 c_0. For example, the proportion of the second oxygen gas in a deposition gas for forming the oxide semiconductor film 108 c_0 is higher than 0% and lower than or equal to 100%, preferably higher than or equal to 10% and lower than or equal to 100%, more preferably higher than or equal to 30% and lower than or equal to 100%.

Note that in the case where the second oxygen gas and an argon gas are used when the oxide semiconductor film 108 c_0 is formed, the flow rate of the argon gas is preferably higher than the flow rate of the second oxygen gas. When the flow rate of the argon gas is set higher, a dense film can be formed as the oxide semiconductor film 108 c_0. Alternatively, the substrate temperature when the oxide semiconductor film 108 c_0 is formed is set high, typically 250° C. or lower, preferably higher than or equal to 150° C. and lower than or equal to 190° C. to form a dense film as the oxide semiconductor film 108 c_0. The dense film formed as the oxide semiconductor film 108 c_0 can suppress entry of a metal element contained in the conductive films 112 a and 112 b into the oxide semiconductor film 108 b_0.

In this embodiment, the oxide semiconductor film 108 b_0 is formed by a sputtering method using an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 [atomic ratio]) and then the oxide semiconductor film 108 c_0 is successively formed in a vacuum by a sputtering method using an In—Ga—Zn metal oxide target (In:Ga:Zn=1:1:1.2 [atomic ratio]). The substrate temperature when the oxide semiconductor film 108 b_0 is formed is set to 170° C., and the substrate temperature when the oxide semiconductor film 108 c_0 is formed is set to 170° C. As the deposition gas for forming the oxide semiconductor film 108 b_0, an oxygen gas at a flow rate of 60 sccm and an argon gas at a flow rate of 140 sccm are used. As the deposition gas for forming the oxide semiconductor film 108 c_0, an oxygen gas at a flow rate of 100 sccm and an argon gas at a flow rate of 100 sccm are used.

Next, the oxide semiconductor film 108 b_0 and the oxide semiconductor film 108 c_0 are processed into desired shapes, so that the island-shaped oxide semiconductor films 108 b and 108 c are formed (see FIGS. 9A and 9B).

Next, a conductive film 112 to be a source electrode and a drain electrode is formed over the insulating film 107 and the oxide semiconductor film 108 by a sputtering method (see FIGS. 9C and 9D).

In this embodiment, the conductive film 112 is formed of a 400-nm-thick aluminum film is stacked over a 50-nm-thick tungsten film by a sputtering method. Although the conductive film 112 have a two-layer structure in this embodiment, one embodiment of the present invention is not limited thereto. For example, the conductive film 112 may have a three-layer structure in which a 400-nm-thick aluminum film is stacked over a 50-nm-thick tungsten film, and a 100-nm-thick titanium film is stacked over the 400-nm-thick aluminum film.

Next, the conductive film 112 is processed into desired shapes, so that the separate conductive films 112 a and 112 b are formed (see FIGS. 9E and 9F).

In this embodiment, the conductive film 112 is processed with a dry etching apparatus. Note that the method for processing the conductive film 112 is not limited thereto, and a wet etching apparatus may be used, for example. When the conductive film 112 is processed, a finer pattern can be formed with a dry etching apparatus than with a wet etching apparatus. On the other hand, the conductive film 112 can be processed with a wet etching apparatus at lower manufacturing cost than with a dry etching apparatus.

After the conductive films 112 a and 112 b are formed, a surface (on the back channel side) of the oxide semiconductor film 108 (specifically, the oxide semiconductor film 108 c) may be cleaned. The cleaning may be performed, for example, using a chemical solution such as phosphoric acid. The cleaning using a chemical solution such as a phosphoric acid can remove impurities (e.g., an element included in the conductive films 112 a and 112 b) attached to the surface of the oxide semiconductor film 108 c. Note that the cleaning is not necessarily performed, and thus the cleaning may be unnecessary.

In the step of forming the conductive films 112 a and 112 b and/or the cleaning step, the thickness of a region of the oxide semiconductor film 108 which is not covered by the conductive films 112 a and 112 b might be reduced.

Next, the insulating films 114 and 116 are formed over the oxide semiconductor film 108 and the conductive films 112 a and 112 b (see FIGS. 10A and 10B).

Note that after the insulating film 114 is formed, the insulating film 116 is preferably formed in succession without exposure to the air. After the insulating film 114 is formed, the insulating film 116 is formed in succession without exposure to the air while at least one of the flow rate of a source gas, pressure, a high-frequency power, and a substrate temperature is adjusted, whereby the concentration of impurities attributed to the atmospheric component at the interface between the insulating film 114 and the insulating film 116 can be reduced and oxygen in the insulating films 114 and 116 can be moved to the oxide semiconductor film 108; accordingly, the amount of oxygen vacancies in the oxide semiconductor film 108 can be reduced.

As the insulating film 114, a silicon oxynitride film can be formed by a PECVD method, for example. In this case, a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. Examples of the oxidizing gas include dinitrogen monoxide and nitrogen dioxide. An insulating film containing nitrogen and having a small number of defects can be formed as the insulating film 114 by a PECVD method under the conditions where the flow rate of the oxidizing gas is higher than 20 times and lower than 100 times, preferably higher than or equal to 40 times and lower than or equal to 80 times, that of the deposition gas; and the pressure in a treatment chamber is lower than 100 Pa, preferably lower than or equal to 50 Pa.

In this embodiment, a silicon oxynitride film is formed as the insulating film 114 by a PECVD method under the conditions where the substrate 102 is held at a temperature of 220° C., silane at a flow rate of 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm are used as a source gas, the pressure in the treatment chamber is 20 Pa, and a high-frequency power of 100 W at 13.56 MHz (1.6×10⁻² W/cm² as the power density) is supplied to parallel-plate electrodes.

As the insulating film 116, a silicon oxide film or a silicon oxynitride film is formed under the following conditions: the substrate placed in a treatment chamber of the PECVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 180° C. and lower than or equal to 350° C.; the pressure is greater than or equal to 100 Pa and less than or equal to 250 Pa, preferably greater than or equal to 100 Pa and less than or equal to 200 Pa with introduction of a source gas into the treatment chamber; and a high-frequency power of greater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm², preferably greater than or equal to 0.25 W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrode provided in the treatment chamber.

As the deposition conditions of the insulating film 116, the high-frequency power having the above power density is supplied to a reaction chamber having the above pressure, whereby the degradation efficiency of the source gas in plasma is increased, oxygen radicals are increased, and oxidation of the source gas is promoted; thus, the oxygen content in the insulating film 116 becomes higher than that in the stoichiometric composition. In addition, in the film formed at a substrate temperature within the above temperature range, the bond between silicon and oxygen is weak, and accordingly, part of oxygen in the film is released by heat treatment in a later step. Thus, it is possible to form an oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition and from which part of oxygen is released by heating.

Note that the insulating film 114 functions as a protective film for the oxide semiconductor film 108 in the step of forming the insulating film 116. Therefore, the insulating film 116 can be formed using the high-frequency power having a high power density while damage to the oxide semiconductor film 108 is reduced.

Note that in the deposition conditions of the insulating film 116, when the flow rate of the deposition gas containing silicon with respect to the oxidizing gas is increased, the amount of defects in the insulating film 116 can be reduced. As a typical example, it is possible to form an oxide insulating film in which the amount of defects is small, i.e., the spin density of a signal which appears at g=2.001 originating from a dangling bond of silicon is lower than 6×10¹⁷ spins/cm³, preferably lower than or equal to 3×10¹⁷ spins/cm³, further preferably lower than or equal to 1.5×10¹⁷ spins/cm³ by ESR measurement. As a result, the reliability of the transistor 100 can be improved.

Heat treatment (hereinafter referred to as first heat treatment) is preferably performed after the insulating films 114 and 116 are formed. The first heat treatment can reduce nitrogen oxide contained in the insulating films 114 and 116. By the first heat treatment, part of oxygen contained in the insulating films 114 and 116 can be moved to the oxide semiconductor film 108, so that the amount of oxygen vacancies included in the oxide semiconductor film 108 can be reduced.

The temperature of the first heat treatment is typically lower than 400° C., preferably lower than 375° C., further preferably higher than or equal to 150° C. and lower than or equal to 350° C. The first heat treatment may be performed under an atmosphere of nitrogen, oxygen, ultra-dry air (air in which a water content is 20 ppm or less, preferably 1 ppm or less, further preferably 10 ppb or less), or a rare gas (argon, helium, and the like). Note that an electric furnace, rapid thermal anneal (RTA), or the like can be used for the heat treatment, in which it is preferable that hydrogen, water, and the like not be contained in the nitrogen, oxygen, ultra-dry air, or a rare gas.

Next, a mask is formed over the insulating film 116 through a lithography process, and an opening portion 152 a is formed in desired regions in the insulating film 114 and 116. Note that the opening portion 152 a is formed to reach the conductive film 112 b (see FIGS. 10C and 10D).

Next, an oxide semiconductor film 120 is formed over the insulating film 116 to cover the opening portion 152 a (see FIGS. 10E and 10F and FIGS. 11A and 11B).

FIGS. 10E and 10F are schematic cross-sectional views showing an inner portion of a deposition apparatus when the oxide semiconductor film 120 is formed over the insulating film 116. In FIGS. 10E and 10F, a sputtering apparatus is used as the deposition apparatus, and a target 193 placed inside the sputtering apparatus and plasma 194 formed under the target 193 are schematically shown.

When the oxide semiconductor film 120 is formed, plasma discharge is performed in an atmosphere containing a third oxygen gas. At this time, oxygen is added to the insulating film 116 over which the oxide semiconductor film 120 is to be formed. When the oxide semiconductor film 120 is formed, an inert gas (e.g., a helium gas, an argon gas, or a xenon gas) and the third oxygen gas may be mixed. For example, it is preferable to use the argon gas and the third oxygen gas with the flow rate higher than the flow rate of the argon gas. When the flow rate of the third oxygen gas is set higher, oxygen can be favorably added to the insulating film 116. As an example of the formation conditions of the oxide semiconductor film 120, the proportion of the third oxygen gas in a whole deposition gas is higher than or equal to 50% and lower than or equal to 100%, preferably higher than or equal to 80% and lower than or equal to 100%.

In FIGS. 10E and 10F, oxygen or excess oxygen added to the insulating film 116 is schematically shown by arrows of broken lines.

The oxide semiconductor film 120 is formed at a substrate temperature higher than or equal to room temperature and lower than 340° C., preferably higher than or equal to room temperature and lower than or equal to 300° C., further preferably higher than or equal to 100° C. and lower than or equal to 250° C., still further preferably higher than or equal to 100° C. and lower than or equal to 200° C. The oxide semiconductor film 120 is formed while being heated, so that the crystallinity of the oxide semiconductor film 120 can be increased. On the other hand, in the case where a large-sized glass substrate (e.g., the 6th generation to the 10th generation) is used as the substrate 102 and the oxide semiconductor film 120 is formed at a substrate temperature higher than or equal to 150° C. and lower than 340° C., the substrate 102 might be changed in shape (distorted or warped). In the case where a large-sized glass substrate is used, the change in the shape of the glass substrate can be suppressed by forming the oxide semiconductor film 120 at a substrate temperature higher than or equal to 100° C. and lower than 150° C.

In this embodiment, the oxide semiconductor film 120 is formed by a sputtering method using an In—Ga—Zn metal oxide target (In:Ga:Zn=1:3:6 [atomic ratio]). The substrate temperature when the oxide semiconductor film 120 is formed is set to 170° C. As the deposition gas for forming the oxide semiconductor film 120, an oxygen gas at a flow rate of 100 sccm is used.

As the oxide semiconductor film 120, the above-described oxide semiconductor film (with an atomic ratio of In:Ga:Zn=1:1:1, In:Ga:Zn=1:3:2, In:Ga:Zn=1:3:4, In:Ga:Zn=1:3:6, In:Ga:Zn=3:1:2, or In:Ga:Zn=4:2:3, for example) may be used.

Next, the oxide semiconductor film 120 is processed into a desired shape, so that the island-shaped oxide semiconductor films 120 a and 120 b are formed (see FIGS. 11C and 11D).

Next, the insulating film 118 is formed over the insulating film 116 and the oxide semiconductor films 120 a and 120 b (see FIGS. 11E and 11F).

The insulating film 118 includes one or both of hydrogen and nitrogen. As the insulating film 118, a silicon nitride film is preferably used, for example. The insulating film 118 can be formed by a sputtering method or a PECVD method, for example. In the case where the insulating film 118 is formed by a PECVD method, for example, the substrate temperature is lower than 400° C., preferably lower than 375° C., further preferably higher than or equal to 180° C. and lower than or equal to 350° C. The substrate temperature at which the insulating film 118 is formed is preferably within the above range because a dense film can be formed. Furthermore, when the substrate temperature at which the insulating film 118 is formed is within the above range, oxygen or excess oxygen in the insulating films 114 and 116 can be moved to the oxide semiconductor film 108.

After the insulating film 118 is formed, heat treatment similar to the first heat treatment (hereinafter referred to as second heat treatment) may be performed. Through such heat treatment at lower than 400° C., preferably lower than 375° C., further preferably higher than or equal to 180° C. and lower than or equal to 350° C. after the addition of oxygen to the insulating film 116 when the oxide semiconductor film 120 is formed, oxygen or excess oxygen in the insulating film 116 can be moved to the oxide semiconductor film 108 (particularly, the oxide semiconductor film 108 b) and compensate oxygen vacancies in the oxide semiconductor film 108.

Oxygen moved to the oxide semiconductor film 108 is described with reference to FIGS. 12A and 12B. FIGS. 12A and 12B are model diagrams illustrating oxygen moved to the oxide semiconductor film 108 due to the substrate temperature at the time of forming the insulating film 118 (typically, lower than 375° C.) or the second heat treatment after the formation of the insulating film 118 (typically, lower than 375° C.). In FIGS. 12A and 12B, oxygen (oxygen radicals, oxygen atoms, or oxygen molecules) in the oxide semiconductor film 108 are shown by arrows of broken lines.

In the oxide semiconductor film 108 in FIGS. 12A and 12B, oxygen vacancies are compensated with oxygen moved from films in contact with the oxide semiconductor film 108 (here, the insulating film 107 and the insulating film 114). Specifically, in the semiconductor device of one embodiment of the present invention, the insulating film 107 includes an excess oxygen region because an oxygen gas is used at the time of forming the oxide semiconductor film 108 b by sputtering and oxygen is added to the insulating film 107. Furthermore, the insulating film 116 includes an excess oxygen region because an oxygen gas is used at the time of forming the oxide semiconductor film 120 by sputtering and oxygen is added to the insulating film 116. In the oxide semiconductor film 108 between the insulating films including the excess oxygen regions, oxygen vacancies can be favorably compensated.

Furthermore, the insulating film 106 is provided under the insulating film 107, and the insulating film 118 is provided over the insulating films 114 and 116. When the insulating films 106 and 118 are formed using a material having low oxygen permeability, e.g., silicon nitride, oxygen contained in the insulating films 107, 114, and 116 can be confined to the oxide semiconductor film 108 side; thus, oxygen can be favorably moved to the oxide semiconductor film 108.

The insulating film 118 includes one or both of hydrogen and nitrogen. Thus, one or both of hydrogen and nitrogen is added to the oxide semiconductor films 120 a and 120 b in contact with the formed insulating film 118, so that the oxide semiconductor films 120 a and 120 b have high carrier density and can function as oxide conductive films.

In the case where a silicon nitride film is formed by a PECVD method as the insulating film 118, a deposition gas containing silicon, nitrogen, and ammonia are preferably used as a source gas. A small amount of ammonia compared to the amount of nitrogen is used, whereby ammonia is dissociated in the plasma and activated species are generated. The activated species break a bond between silicon and hydrogen that are contained in a deposition gas containing silicon and a triple bond between nitrogen molecules. As a result, a dense silicon nitride film having few defects, in which bonds between silicon and nitrogen are promoted and bonds between silicon and hydrogen is few, can be formed. On the other hand, when the amount of ammonia with respect to nitrogen is large, decomposition of a deposition gas containing silicon and decomposition of nitrogen are not promoted, so that a sparse silicon nitride film in which bonds between silicon and hydrogen remain and defects are increased is formed. Therefore, in the source gas, the flow rate of nitrogen is set to be preferably 5 times or more and 50 times or less, more preferably 10 times or more and 50 times or less the flow rate of ammonia.

In this embodiment, with the use of a PECVD apparatus, a 50-nm-thick silicon nitride film is formed as the insulating film 118 using silane, nitrogen, and ammonia as a source gas. The flow rate of silane is 50 sccm, the flow rate of nitrogen is 5000 sccm, and the flow rate of ammonia is 100 sccm. The pressure in the treatment chamber is 100 Pa, the substrate temperature is 350° C., and high-frequency power of 1000 W is supplied to parallel-plate electrodes with a 27.12 MHz high-frequency power source. Note that the PECVD apparatus is a parallel-plate PECVD apparatus in which the electrode area is 6000 cm², and the power per unit area (power density) into which the supplied power is converted is 1.7×10⁻¹ W/cm².

Note that in this embodiment, hydrogen or nitrogen is added from the insulating film 118 to the oxide semiconductor films 120 a and 120 b to increase the carrier density of the oxide semiconductor films 120 a and 120 b; however, a method for increasing the carrier density of the oxide semiconductor films 120 a and 120 b is not limited thereto. For example, treatment for adding an impurity element to the oxide semiconductor films 120 a and 120 b may be performed to increase the carrier density of the oxide semiconductor films 120 a and 120 b.

Typical examples of the impurity element are hydrogen, boron, carbon, nitrogen, fluorine, aluminum, silicon, phosphorus, chlorine, and rare gas elements. Typical examples of rare gas elements are helium, neon, argon, krypton, and xenon. When the impurity element is added to the oxide semiconductor film, a bond between a metal element and oxygen in the oxide semiconductor film is cut, whereby an oxygen vacancy is formed. Alternatively, when the impurity element is added to the oxide semiconductor film, oxygen bonded to a metal element in the oxide semiconductor film is bonded to the impurity element, and the oxygen is released from the metal element, whereby an oxygen vacancy is formed. As a result, carrier density is increased in the oxide semiconductor film, and the oxide semiconductor film has higher conductivity.

Through the above process, the transistor 100 illustrated in FIGS. 1C and 1D can be manufactured.

In the entire manufacturing process of the transistor 100, the substrate temperature is preferably lower than 400° C., further preferably lower than 375° C., still further preferably higher than or equal to 180° C. and lower than or equal to 350° C. because the change in shape of the substrate (distortion or warp) can be reduced even when a large-sized substrate is used. As typical examples of a step in which the substrate temperature is increased in the manufacturing process of the transistor 100, the following are given: the substrate temperature in the formation of the insulating films 106 and 107 (lower than 400° C., preferably higher than or equal to 250° C. and lower than or equal to 350° C.), the substrate temperature in the formation of the oxide semiconductor film 108 (higher than or equal to room temperature and lower than 340° C., preferably higher than or equal to 100° C. and lower than or equal to 200° C., further preferably higher than or equal to 100° C. and lower than 150° C.), the substrate temperature in the formation of the insulating films 116 and 118 (lower than 400° C., preferably lower than 375° C., further preferably higher than or equal to 180° C. and lower than or equal to 350° C.), and the first heat treatment or the second heat treatment (lower than 400° C., preferably lower than 375° C., further preferably higher than or equal to 180° C. and lower than or equal to 350° C.).

The structure and method described in this embodiment can be implemented by being combined as appropriate with any of the other structures and methods described in the other embodiments.

Embodiment 2

In this embodiment, a structure of an oxide semicondutor and the like are described with reference to FIGS. 17A to 17D, FIGS. 18A to 18D, FIGS. 19A to 19C, FIGS. 20A and 20B, FIG. 21, FIG. 22, FIGS. 23A to 23C, FIGS. 24A to 24F, FIGS. 25A to 25G, and FIGS. 26A to 26G.

<2-1. Structure of Oxide Semiconductor>

An oxide semiconductor is classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of a non-single-crystal oxide semiconductor include a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor.

From another perspective, an oxide semiconductor is classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor. Examples of a crystalline oxide semiconductor include a single crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and an nc-OS.

It is known that an amorphous structure is generally defined as being metastable and unfixed, and being isotropic and having no non-uniform structure. In other words, an amorphous structure has a flexible bond angle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot be regarded as a completely amorphous oxide semiconductor. Moreover, an oxide semiconductor that is not isotropic (e.g., an oxide semiconductor that has a periodic structure in a microscopic region) cannot be regarded as a completely amorphous oxide semiconductor. Note that an a-like OS has a periodic structure in a microscopic region, but at the same time has a void and has an unstable structure. For this reason, an a-like OS has physical properties similar to those of an amorphous oxide semiconductor.

As the oxide semiconductor film of one embodiment of the present invention, a CAAC-OS is particularly preferable among the above-described oxide semiconductors. When a CAAC-OS is used as the oxide semiconductor film, the crystallinity of the oxide semiconductor film can be increased, and impurities, oxygen vacancies, or the density of defect states in the oxide semiconductor film can be reduced.

<2-2. CAAC-OS>

First, a CAAC-OS will be described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axis aligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of a CAAC-OS, which is obtained using a TEM, a plurality of pellets can be observed. However, in the high-resolution TEM image, a boundary between pellets, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur.

A CAAC-OS observed with TEM is described below. FIG. 17A shows a high-resolution TEM image of a cross section of the CAAC-OS which is observed from a direction substantially parallel to the sample surface. The high-resolution TEM image is obtained with a spherical aberration corrector function. The high-resolution TEM image obtained with a spherical aberration corrector function is particularly referred to as a Cs-corrected high-resolution TEM image. The Cs-corrected high-resolution TEM image can be obtained with, for example, an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 17B is an enlarged Cs-corrected high-resolution TEM image of a region (1) in FIG. 17A. FIG. 17B shows that metal atoms are arranged in a layered manner in a pellet. Each metal atom layer has a configuration reflecting unevenness of a surface over which the CAAC-OS is formed (hereinafter, the surface is referred to as a formation surface) or the top surface of the CAAC-OS, and is arranged parallel to the formation surface or the top surface of the CAAC-OS.

As shown in FIG. 17B, the CAAC-OS has a characteristic atomic arrangement. The characteristic atomic arrangement is denoted by an auxiliary line in FIG. 17C. FIGS. 17B and 17C prove that the size of a pellet is approximately 1 nm to 3 nm, and the size of a space caused by tilt of the pellets is approximately 0.8 nm. Therefore, the pellet can also be referred to as a nanocrystal (nc). Furthermore, the CAAC-OS can also be referred to as an oxide semiconductor including c-axis aligned nanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, the schematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120 is illustrated by such a structure in which bricks or blocks are stacked (see FIG. 17D). The part in which the pellets are tilted as observed in FIG. 17C corresponds to a region 5161 shown in FIG. 17D.

FIG. 18A shows a Cs-corrected high-resolution TEM image of a plane of the CAAC-OS observed from a direction substantially perpendicular to the sample surface. FIGS. 18B, 18C, and 18D are enlarged Cs-corrected high-resolution TEM images of regions (1), (2), and (3) in FIG. 18A, respectively. FIGS. 18B, 18C, and 18D indicate that metal atoms are arranged in a triangular, quadrangular, or hexagonal configuration in a pellet. However, there is no regularity of arrangement of metal atoms between different pellets.

Next, a CAAC-OS analyzed by XRD is described. For example, when the structure of a CAAC-OS including an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appears at a diffraction angle (2θ) of around 31° as shown in FIG. 19A. This peak is assigned to the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-plane method, another peak may appear when 2θ is around 36°, in addition to the peak at 2θ of around 31°. The peak at 2θ of around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS. It is preferable that in the CAAC-OS analyzed by an out-of-plane method, a peak appear when 2θ is around 31° and that a peak not appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-plane method in which an X-ray beam is incident on a sample in a direction substantially perpendicular to the c-axis, a peak appears when 2θ is around 56°. This peak is attributed to the (110) plane of the InGaZnO₄ crystal. In the case of the CAAC-OS, when analysis (φ scan) is performed with 2θ fixed at around 56° and with the sample rotated using a normal vector of the sample surface as an axis (φ axis), as shown in FIG. 19B, a peak is not clearly observed. In contrast, in the case of a single crystal oxide semiconductor of InGaZnO₄, when φ scan is performed with 2θ fixed at around 56°, as shown in FIG. 19C, six peaks which are assigned to crystal planes equivalent to the (110) plane are observed. Accordingly, the structural analysis using XRD shows that the directions of the a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. For example, when an electron beam with a probe diameter of 300 nm is incident on a CAAC-OS including an InGaZnO₄ crystal in a direction parallel to the sample surface, a diffraction pattern (also referred to as a selected-area transmission electron diffraction pattern) shown in FIG. 20A can be obtained. In this diffraction pattern, spots assigned to the (009) plane of an InGaZnO₄ crystal are included. Thus, the electron diffraction also indicates that pellets included in the CAAC-OS have c-axis alignment and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. Meanwhile, FIG. 20B shows a diffraction pattern obtained in such a manner that an electron beam with a probe diameter of 300 nm is incident on the same sample in a direction perpendicular to the sample surface. As shown in FIG. 20B, a ring-like diffraction pattern is observed. Thus, the electron diffraction also indicates that the a-axes and b-axes of the pellets included in the CAAC-OS do not have regular alignment. The first ring in FIG. 20B is considered to be assigned to the (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal. The second ring in FIG. 20B is considered to be assigned to the (110) plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with high crystallinity. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has small amounts of impurities and defects (e.g., oxygen vacancies).

Note that the impurity means an element other than the main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. For example, an element (specifically, silicon or the like) having higher strength of bonding to oxygen than a metal element included in an oxide semiconductor extracts oxygen from the oxide semiconductor, which results in disorder of the atomic arrangement and reduced crystallinity of the oxide semiconductor. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities or defects might be changed by light, heat, or the like. Impurities contained in the oxide semiconductor might serve as carrier traps or carrier generation sources, for example. Furthermore, oxygen vacancies in the oxide semiconductor serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies is an oxide semiconductor with low carrier density (specifically, lower than 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, further preferably lower than 1×10¹⁹/cm³, and is higher than or equal to 1×10⁻⁹/cm³). Such an oxide semiconductor is referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. A CAAC-OS has a low impurity concentration and a low density of defect states. Thus, the CAAC-OS can be referred to as an oxide semiconductor having stable characteristics.

<2-3. nc-OS>

Next, an nc-OS will be described.

An nc-OS has a region in which a crystal part is observed and a region in which a crystal part is not clearly observed in a high-resolution TEM image. In most cases, the size of a crystal part included in the nc-OS is greater than or equal to 1 nm and less than or equal to 10 nm, or greater than or equal to 1 nm and less than or equal to 3 nm. Note that an oxide semiconductor including a crystal part whose size is greater than 10 nm and less than or equal to 100 nm is sometimes referred to as a microcrystalline oxide semiconductor. In a high-resolution TEM image of the nc-OS, for example, a grain boundary is not clearly observed in some cases. Note that there is a possibility that the origin of the nanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may be referred to as a pellet in the following description.

In the nc-OS, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different pellets in the nc-OS. Thus, the orientation of the whole film is not ordered. Accordingly, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor, depending on an analysis method. For example, when the nc-OS is analyzed by an out-of-plane method using an X-ray beam having a diameter larger than the size of a pellet, a peak indicating a crystal plane does not appear. Furthermore, a diffraction pattern like a halo pattern is observed when the nc-OS is subjected to electron diffraction using an electron beam with a probe diameter (e.g., 50 nm or larger) that is larger than the size of a pellet. Meanwhile, spots appear in a nanobeam electron diffraction pattern of the nc-OS when an electron beam having a probe diameter close to or smaller than the size of a pellet is applied. Moreover, in a nanobeam electron diffraction pattern of the nc-OS, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS, a plurality of spots is shown in a ring-like region in some cases.

Since there is no regularity of crystal orientation between the pellets (nanocrystals) as mentioned above, the nc-OS can also be referred to as an oxide semiconductor including random aligned nanocrystals (RANC) or an oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as compared with an amorphous oxide semiconductor. Therefore, the nc-OS is likely to have a lower density of defect states than an a-like OS and an amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS.

<2-4. A-Like OS>

An a-like OS has a structure intermediate between those of the nc-OS and the amorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed. Furthermore, in the high-resolution TEM image, there are a region where a crystal part is clearly observed and a region where a crystal part is not observed.

The a-like OS has an unstable structure because it includes a void. To verify that an a-like OS has an unstable structure as compared with a CAAC-OS and an nc-OS, a change in structure caused by electron irradiation is described below.

An a-like OS (referred to as Sample A), an nc-OS (referred to as Sample B), and a CAAC-OS (referred to as Sample C) are prepared as samples subjected to electron irradiation. Each of the samples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample is obtained. The high-resolution cross-sectional TEM images show that all the samples have crystal parts.

Note that which part is regarded as a crystal part is determined as follows. It is known that a unit cell of an InGaZnO₄ crystal has a structure in which nine layers including three In—O layers and six Ga—Zn—O layers are stacked in the c-axis direction. The distance between the adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to be 0.29 nm from crystal structural analysis. Accordingly, a portion where the lattice spacing between lattice fringes is greater than or equal to 0.28 nm and less than or equal to 0.30 nm is regarded as a crystal part of InGaZnO₄. Each of lattice fringes corresponds to the a-b plane of the InGaZnO₄ crystal.

FIG. 21 shows change in the average size of crystal parts (at 22 points to 45 points) in each sample. Note that the crystal part size corresponds to the length of a lattice fringe. FIG. 21 indicates that the crystal part size in the a-like OS increases with an increase in the cumulative electron dose. Specifically, as shown by (1) in FIG. 21, a crystal part of approximately 1.2 nm (also referred to as an initial nucleus) at the start of TEM observation grows to a size of approximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OS shows little change from the start of electron irradiation to a cumulative electron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by (2) and (3) in FIG. 21, the average crystal sizes in an nc-OS and a CAAC-OS are approximately 1.4 nm and approximately 2.1 nm, respectively, regardless of the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is induced by electron irradiation. In contrast, in the nc-OS and the CAAC-OS, growth of the crystal part is hardly induced by electron irradiation. Therefore, the a-like OS has an unstable structure as compared with the nc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS because it includes a void. Specifically, the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single crystal oxide semiconductor having the same composition. The density of each of the nc-OS and the CAAC-OS is higher than or equal to 92.3% and lower than 100% of the density of the single crystal oxide semiconductor having the same composition. Note that it is difficult to deposit an oxide semiconductor having a density of lower than 78% of the density of the single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with a rhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm³ and lower than 5.9 g/cm³. For example, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of each of the nc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lower than 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having a certain composition cannot exist in a single crystal structure. In that case, single crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate density equivalent to that of a single crystal oxide semiconductor with the desired composition. The density of a single crystal oxide semiconductor having the desired composition can be calculated using a weighted average according to the combination ratio of the single crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single crystal oxide semiconductors as possible to calculate the density.

As described above, oxide semiconductors have various structures and various properties. Note that an oxide semiconductor may be a stacked layer including two or more of an amorphous oxide semiconductor, an a-like OS, an nc-OS, and a CAAC-OS, for example.

<2-5. Formation Method of CAAC-OS>

An example of a method for forming CAAC-OS will be described below.

FIG. 22 is a schematic view of the inside of a film formation chamber. The CAAC-OS can be formed by a sputtering method.

As shown in FIG. 22, a substrate 5220 and a target 5230 are arranged to face each other. Plasma 5240 is generated between the substrate 5220 and the target 5230. A heating mechanism 5260 is under the substrate 5220. The target 5230 is attached to a backing plate (not illustrated in the drawing). A plurality of magnets is arranged to face the target 5230 with the backing plate positioned therebetween. A sputtering method in which the deposition speed is increased by utilizing a magnetic field of magnets is referred to as a magnetron sputtering method.

The distance d between the substrate 5220 and the target 5230 (also referred to as a target-substrate distance (T-S distance)) is greater than or equal to 0.01 m and less than or equal to 1 m, preferably greater than or equal to 0.02 m and less than or equal to 0.5 m. The deposition chamber is mostly filled with a deposition gas (e.g., an oxygen gas, an argon gas, or a mixed gas containing oxygen at 5 vol % or higher) and the pressure in the deposition chamber is controlled to be higher than or equal to 0.01 Pa and lower than or equal to 100 Pa, preferably higher than or equal to 0.1 Pa and lower than or equal to 10 Pa. Here, discharge starts by application of a voltage at a certain value or higher to the target 5230, and the plasma 5240 is observed. The magnetic field forms a high-density plasma region in the vicinity of the target 5230. In the high-density plasma region, the deposition gas is ionized, so that an ion 5201 is generated. Examples of the ion 5201 include an oxygen cation (O⁺) and an argon cation (Ar⁺).

Here, the target 5230 has a polycrystalline structure which includes a plurality of crystal grains and in which a cleavage plane exists in any of the crystal grains. FIGS. 23A to 23C show a crystal structure of InMZnO₄ (the element M is Al, Ga, Y, or Sn, for example) included in the target 5230 as an example. Note that the crystal structure shown in FIG. 23A is InMZnO₄ observed from a direction parallel to a b-axis. In the crystal of InMZnO₄, oxygen atoms are negatively charged, whereby repulsive force is generated between the two adjacent M-Zn—O layers. Thus, the InMZnO₄ crystal has a cleavage plane between the two adjacent M-Zn—O layers.

The ion 5201 generated in the high-density plasma region is accelerated toward the target 5230 side by an electric field, and then collides with the target 5230. At this time, the pellet 5200 which is a flat-plate-like or pellet-like sputtered particle is separated from the cleavage plane (see FIG. 22). The pellet 5200 corresponds to a portion between the two cleavage planes shown in FIG. 23A. Thus, when the pellet 5200 is observed, the cross-section thereof is as shown in FIG. 23B, and the top surface thereof is as shown in FIG. 23C. Note that the structure of the pellet 5200 may be distorted by an impact of collision of the ion 5201.

The pellet 5200 is a flat-plate-like (pellet-like) sputtered particle having a triangle plane, e.g., regular triangle plane. Alternatively, the pellet 5200 is a flat-plate-like (pellet-like) sputtered particle having a hexagon plane, e.g., regular hexagon plane. However, the shape of a flat plane of the pellet 5200 is not limited to a triangle or a hexagon. For example, the flat plane may have a shape formed by combining two or more triangles. For example, a quadrangle (e.g., rhombus) may be formed by combining two triangles (e.g., regular triangles).

The thickness of the pellet 5200 is determined depending on the kind of deposition gas and the like. For example, the thickness of the pellet 5200 is greater than or equal to 0.4 nm and less than or equal to 1 nm, preferably greater than or equal to 0.6 nm and less than or equal to 0.8 nm. In addition, the width of the pellet 5200 is, for example, greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 2 nm and less than or equal to 50 nm, more preferably greater than or equal to 3 nm and less than or equal to 30 nm. For example, the ion 5201 collides with the target 5230 including the In-M-Zn oxide. Then, the pellet 5200 including three layers of an M-Zn—O layer, an In—O layer, and an M-Zn—O layer is separated. Note that along with the separation of the pellet 5200, a particle 5203 is also sputtered from the target 5230. The particle 5203 has an atom or an aggregate of several atoms. Therefore, the particle 5203 can be referred to as an atomic particle.

The pellet 5200 may receive a charge when passing through the plasma 5240, so that surfaces thereof are negatively or positively charged. For example, the pellet 5200 receives a negative charge from O²⁻ in the plasma 5240. As a result, oxygen atoms on the surfaces of the pellet 5200 may be negatively charged. In addition, when passing through the plasma 5240, the pellet 5200 is sometimes combined with indium, the element M, zinc, oxygen, or the like in the plasma 5240 to grow up.

The pellet 5200 and the particle 5203 that have passed through the plasma 5240 reach the surface of the substrate 5220. Note that part of the particle 5203 is sometimes discharged to the outside by a vacuum pump or the like because the particle 5203 is small in mass.

Next, deposition of the pellet 5200 and the particle 5203 over the surface of the substrate 5220 is described with reference to FIGS. 24A to 24F.

First, a first of the pellets 5200 is deposited over the substrate 5220. Since the pellet 5200 has a flat-plate-like shape, it is deposited so that the flat plane faces the surface of the substrate 5220. Here, a charge on a surface of the pellet 5200 on the substrate 5220 side is lost through the substrate 5220.

Next, a second of the pellets 5200 reaches the substrate 5220. Since a surface of the first pellet 5200 and a surface of the second pellet 5200 are charged, they repel each other. As a result, the second pellet 5200 avoids being deposited over the first pellet 5200, and is deposited with its flat plane facing the surface of the substrate 5220 so as to be a little distance away from the first pellet 5200. With repetition of this, millions of the pellets 5200 are deposited over the surface of the substrate 5220 to have a thickness of one layer. A region where no pellet 5200 is deposited is generated between adjacent pellets 5200 (see FIG. 24A).

Then, the particles 5203 that have received energy from plasma reach the surface of the substrate 5220. The particles 5203 cannot be deposited on an active region such as the surfaces of the pellets 5200. For this reason, the particles 5203 move to regions where no pellet 5200 is deposited and are attached to side surfaces of the pellets 5200. Since available bonds of the particles 5203 are activated by energy received from plasma, the particles 5203 are chemically bonded to the pellets 5200 to form lateral growth portions 5202 (see FIG. 24B).

The lateral growth portions 5202 then further grow laterally so that the pellets 5200 are anchored to each other (see FIG. 24C). In this manner, the lateral growth portions 5202 are formed until they fill regions where no pellet 5200 is deposited. This mechanism is similar to a deposition mechanism for an atomic layer deposition (ALD) method.

Even when the deposited pellets 5200 are oriented in different directions, the particles 5203 cause a lateral growth to fill gaps between the pellets 5200; thus, no clear grain boundary is formed. In addition, as the particles 5203 make a smooth connection between the pellets 5200, a crystal structure different from single crystal and polycrystal structures is formed. In other words, a crystal structure including distortion between minute crystal regions (pellets 5200) is formed. Regions filling the gaps between the crystal regions are distorted crystal regions, and thus, it will be not appropriate to say that the regions have an amorphous structure.

Next, new pellets 5200 are deposited with their flat planes facing the surface (see FIG. 24D). After that, the particles 5203 are deposited so as to fill regions where no pellet 5200 is deposited, thereby forming the lateral growth portions 5202 (see FIG. 24E). In such a manner, the particles 5203 are attached to side surfaces of the pellets 5200 and the lateral growth portions 5202 cause a lateral growth so that the pellets 5200 in the second layer are anchored to each other (see FIG. 24F). Deposition continues until the m-th layer (m is an integer of two or more) is formed; as a result, a stacked-layer thin film structure is formed.

A deposition way of the pellets 5200 changes according to the surface temperature of the substrate 5220 or the like. For example, if the surface temperature of the substrate 5220 is high, migration of the pellets 5200 occurs over the surface of the substrate 5220. As a result, a proportion of the pellets 5200 that are directly connected with each other without the particles 5203 increases, whereby a CAAC-OS with high orientation is made. The surface temperature of the substrate 5220 for formation of the CAAC-OS is higher than or equal to room temperature and lower than 340° C., preferably higher than or equal to room temperature and lower than or equal to 300° C., further preferably higher than or equal to 100° C. and lower than or equal to 250° C., still further preferably higher than or equal to 100° C. and lower than or equal to 200° C. Therefore, even when a large-sized substrate of the 8th generation or more is used as the substrate 5220, a warp or the like due to the deposition of the CAAC-OS hardly occurs.

In contrast, if the surface temperature of the substrate 5220 is low, the migration of the pellets 5200 over the substrate 5220 does not easily occur. As a result, the pellets 5200 are stacked to form an nc-OS or the like with low orientation. In the nc-OS, the pellets 5200 are possibly deposited with certain gaps because the pellets 5200 are negatively charged. Therefore, the nc-OS has low orientation but some regularity, and thus it has a denser structure than an amorphous oxide semiconductor.

When spaces between pellets are extremely small in a CAAC-OS, the pellets may form a large pellet. The inside of the large pellet has a single crystal structure. For example, the size of the pellet may be greater than or equal to 10 nm and less than or equal to 200 nm, greater than or equal to 15 nm and less than or equal to 100 nm, or greater than or equal to 20 nm and less than or equal to 50 nm, when seen from the above.

The pellets are considered to be deposited on the surface of the substrate according to such a deposition model. A CAAC-OS can be deposited even when a formation surface does not have a crystal structure. This indicates that the above-described deposition model, which is a growth mechanism different from an epitaxial growth, has high validity. In addition, with the above-described deposition model, a uniform film of a CAAC-OS or an nc-OS can be formed even over a large-sized glass substrate or the like. Even when the surface of the substrate (formation surface) has an amorphous structure (e.g., amorphous silicon oxide), for example, a CAAC-OS can be formed.

In addition, even when the surface of the substrate (formation surface) has an uneven shape, the pellets are aligned along the shape.

The above-described deposition model suggests that a CAAC-OS with high crystallinity can be formed in the following manner: deposition is performed in a high vacuum to have a long mean free path, plasma energy is weakened to reduce damage around a substrate, and thermal energy is applied to a formation surface to repair damage due to plasma during deposition.

The above-described deposition model can be used not only for the case where a target has a polycrystalline structure of a composite oxide with a plurality of crystal grains, such as an In-M-Zn oxide, and any of the crystal grains have a cleavage plane; but also for the case where, for example, a target of a mixture containing indium oxide, an oxide of the element M, and zinc oxide is used.

Since there is no cleavage plane in a target of a mixture, atomic particles are separated from the target by sputtering. During deposition, a high electric field region of plasma is formed around a target. Because of the high electric field region of plasma, atomic particles separated from the target are anchored to each other to cause a lateral growth. For example, indium atoms, which are atomic particles, are anchored to each other and cause a lateral growth to be a nanocrystal formed of an In—O layer, and then an M-Zn—O layer is bonded above and below the nanocrystalline In—O layer so as to complement the nanocrystalline In—O layer. In this manner, a pellet can be formed even when a target of a mixture is used. Accordingly, the above-described deposition model can also be applied to the case of using a target of a mixture.

Note that in the case where a high electric field region of plasma is not formed around a target, only atomic particles separated from the targets are deposited on a substrate surface. In that case, a lateral growth of an atomic particle might occur on the substrate surface. However, since the orientations of atomic particles are not the same, the crystal orientation in the resulting thin film is not uniform. As a result, an nc-OS or the like is obtained.

<2-6. Lateral Growth>

The following description explains that a lateral growth occurs when the particles 5203 are attached to (bonded to or adsorbed on) the pellet 5200 laterally.

FIGS. 25A to 25E illustrate a structure of the pellet 5200 and positions to which metal ions can be attached. A model assumed as the pellet 5200 is a cluster model with 84 atoms extracted from an InMZnO₄ crystal structure with a constant stoichiometric composition. The case where the element M is Ga is described below. FIG. 25F illustrates a structure of the pellet 5200 seen in the direction parallel to the c-axis. FIG. 25G illustrates a structure of the pellet 5200 seen in the direction parallel to the a-axis.

The positions to which metal ions can be attached are represented as a position A, a position B, a position a, a position b, and a position c. The position A is an upper part of an interstitial site surrounded by one gallium atom and two zinc atoms on the top surface of the pellet 5200. The position B is an upper part of an interstitial site surrounded by two gallium atoms and one zinc atom on the top surface of the pellet 5200. The position a is in an indium site on a side surface of the pellet 5200. The position b is in an interstitial site between an In—O layer and a Ga—Zn—O layer on a side surface of the pellet 5200. The position c is in a gallium site on a side surface of the pellet 5200.

The relative energy was estimated from first principles calculation in each case where a metal ion was located in the assumed position (the position A, the position B, the position a, the position b, or the position c). In the calculation, first principles calculation software VASP (Vienna Ab initio Simulation Package) was used. For the exchange-correlation potential, Perdew-Burke-Ernzerhof (PBE) type generalized gradient approximation (GGA) was used, and for the ion potential, a projector augmented wave (PAW) method was used. The cut-off energy was 400 eV, and F-only k-point sampling was used. Table 2 shows the relative energies in the case where an indium ion (In³⁺), a gallium ion (Ga³⁺), and a zinc ion (Zn²⁺) are located at the position A, the position B, the position a, the position b, and the position c. Note that the relative energy is a relative value under the condition where the energy of the model with the lowest energy among the calculated models is set to 0 eV.

TABLE 2 Relative energy [eV] Top surface of pellet Side surface of pellet Ion A B a b c In³⁺ 2.1 1.5 0.0 1.8 1.9 Ga³⁺ 3.7 3.0 0.6 0.0 3.5 Zn²⁺ 2.3 1.8 0.0 0.6 2.9

It is found that any metal ion is more likely to be attached to the side surface of the pellet 5200 than to the top surface thereof. It is also found that a zinc ion as well as an indium ion is most likely to be attached to the indium site at the position a.

Ease of an oxygen ion (O²⁻) attached to the pellet 5200 was examined. FIGS. 26A to 26E illustrate a structure of the pellet 5200 and positions to which oxygen ions can be attached. FIG. 26F illustrates a structure of the pellet 5200 seen in the direction parallel to the c-axis. FIG. 26G illustrates a structure of the pellet 5200 seen in the direction parallel to the b-axis.

The positions to which oxygen ions can be attached are represented as a position C, a position D, a position d, a position e, and a position f. In the position C, an oxygen ion is bonded to gallium on the top surface of the pellet 5200. In the position D, an oxygen ion is bonded to zinc on the top surface of the pellet 5200. In the position d, an oxygen ion is bonded to indium on a side surface of the pellet 5200. In the position e, an oxygen ion is bonded to gallium on a side surface of the pellet 5200. In the position f, an oxygen ion is bonded to zinc on a side surface of the pellet 5200.

The relative energy was estimated from first principles calculation in each case where an oxygen ion was located in the assumed position (the position C, the position D, the position d, the position e, or the position f). Table 3 shows the relative energies in the case where oxygen ions (O²⁻) are located at the position C, the position D, the position d, the position e, and the position f.

TABLE 3 Relative energy [eV] Top surface of pellet Side surface of pellet Ion C D d e f O²⁻ 3.9 3.6 0.0 0.5 1.5

It is found that the oxygen ion is also likely to be attached to the side surface of the pellet 5200 than to the top surface thereof.

According to the above, the particle 5203 that has approached the pellet 5200 is preferentially attached to the side surface of the pellet 5200. This suggests that the deposition model in which a lateral growth of the pellet 5200 occurs when the particles 5203 are attached to the side surface of the pellet 5200 has high validity.

Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments or examples.

Embodiment 3

In this embodiment, a display device including a semiconductor device of one embodiment of the present invention is described with reference to FIGS. 27A and 27B and FIGS. 28 to 45. Note that a structure in which a liquid crystal element is used as a display element of a display device (a liquid crystal display device) is specifically described in this embodiment.

3-1. Liquid Crystal Display Device

A liquid crystal display device 880 in FIG. 27A includes a pixel portion 871, a gate driver 874, a source driver 876, m scan lines 877 which are arranged parallel or substantially parallel to each other and whose potentials are controlled by the gate driver 874, and n signal lines 879 which are arranged parallel or substantially parallel to each other and whose potentials are controlled by the source driver 876. Further, the pixel portion 871 includes a plurality of pixels 870 arranged in a matrix. Furthermore, common lines 875 arranged parallel or substantially parallel to each other are provided along the signal lines 879. The gate driver 874 and the source driver 876 are collectively referred to as a driver circuit portion in some cases.

Each of the scan lines 877 is electrically connected to the n pixels 870 arranged in the corresponding row among the plurality of pixels 870 arranged in m rows and n columns in the pixel portion 871. Each of the signal lines 879 is electrically connected to the m pixels 870 arranged in the corresponding column among the plurality of pixels 870 arranged in m rows and n columns. Note that m and n are each an integer of 1 or more. Each of the common lines 875 is electrically connected to the m pixels 870 arranged in the corresponding row among the pixels 870 arranged in m rows and n columns.

FIG. 27B illustrates an example of a circuit structure that can be used for the pixel 870 in the liquid crystal display device 880 illustrated in FIG. 27A.

The pixel 870 illustrated in FIG. 27B includes a liquid crystal element 851, a transistor 852, and a capacitor 855.

The transistor described in Embodiment 1 can be used as the transistor 852.

One of a pair of electrodes of the liquid crystal element 851 is connected to the transistor 852 and the potential thereof is set according to the specifications of the pixel 870 as appropriate. The other of the pair of electrodes of the liquid crystal element 851 is connected to the common line 875 and a common potential is applied thereto. The alignment state of liquid crystals in the liquid crystal element 851 is controlled in accordance with data written to the transistor 852.

The liquid crystal element 851 is an element which controls transmission or non-transmission of light utilizing an optical modulation action of liquid crystal. Note that optical modulation action of a liquid crystal is controlled by an electric field applied to the liquid crystal (including a horizontal electric field, a vertical electric field, and an oblique electric field). As the liquid crystal used for the liquid crystal element 851, a liquid crystal material such as thermotropic liquid crystal, low-molecular liquid crystal, high-molecular liquid crystal, polymer dispersed liquid crystal, ferroelectric liquid crystal, or anti-ferroelectric liquid crystal can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric field mode, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition which includes liquid crystal exhibiting a blue phase and a chiral material has a short response time and has optical isotropy. In addition, the liquid crystal composition which includes liquid crystal exhibiting a blue phase does not need alignment treatment and has a small viewing angle dependence. An alignment film does not need to be provided and rubbing treatment is thus not necessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device in the manufacturing process can be reduced.

As a driving method of the liquid crystal display device 880 including the liquid crystal element 851, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be used.

The liquid crystal display device 880 may be a normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode. As a vertical alignment mode, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, or an ASV mode can be employed, for example.

3-2. Horizontal Electric Field Mode Liquid Crystal Display Device

First, liquid crystal display devices using a horizontal electric field mode, typically, liquid crystal display devices using an FFS mode and an IPS mode are described.

In the structure of the pixel 870 illustrated in FIG. 27B, one of a source electrode and a drain electrode of the transistor 852 is electrically connected to the signal line 879, and the other is electrically connected to the one of the pair of electrodes of the liquid crystal element 851. A gate electrode of the transistor 852 is electrically connected to the scan line 877. The transistor 852 has a function of controlling whether to write a data signal.

In the structure of the pixel 870 shown in FIG. 27B, one of a pair of electrodes of the capacitor 855 is connected to the other of the source electrode and the drain electrode of the transistor 852. The other of the pair of electrodes of the capacitor 855 is electrically connected to the common line 875. The potential of the common line 875 is set in accordance with the specifications of the pixel 870 as appropriate. The capacitor 855 functions as a storage capacitor for retaining written data. Note that in the liquid crystal display device 880 driven by the FFS mode, the one of the pair of electrodes of the capacitor 855 is partly or entirely the one of the pair of electrodes of the liquid crystal element 851, and the other of the pair of electrodes of the capacitor 855 is partly or entirely the other of the pair of electrodes of the liquid crystal element 851.

3-3. Structure Example 1 of Horizontal Electric Field Element Substrate

A specific structure of an element substrate included in the liquid crystal display device 880 is described. FIG. 28 is a top view of a plurality of pixels 870 a, 870 b, and 870 c included in the liquid crystal display device 880 driven by the FFS mode.

In FIG. 28, a conductive film 813 functioning as a scan line extends substantially perpendicularly to the signal line (in the horizontal direction in the drawing). A conductive film 821 a functioning as a signal line extends substantially perpendicularly to the scan line (in the vertical direction in the drawing). As the conductive film 813 functioning as a scan line is electrically connected to the gate driver 874, and the conductive film 821 a functioning as a signal line is electrically connected to the source driver 876 (see FIG. 27A).

The transistor 852 is provided in the vicinity of the intersection portion of the scan line and the signal line. The transistor 852 includes: the conductive film 813 functioning as a gate electrode; the gate insulating film (not illustrated in FIG. 28); an oxide semiconductor film 808 where a channel region is formed, over the gate insulating film; and the conductive film 821 a and a conductive film 821 b that function as a source electrode and a drain electrode. The conductive film 813 also functions as a scan line, and a region of the conductive film 813 that overlaps with the oxide semiconductor film 808 functions as the gate electrode of the transistor 852. The conductive film 821 a functions as a signal line, and the conductive film 821 a that overlaps with the oxide semiconductor film 808 serves as the source electrode or the drain electrode of the transistor 852. Furthermore, in the top view shown in FIG. 28, an end portion of the scan line is located on the outer side than an end portion of the oxide semiconductor film 808. Thus, the scan line functions as a light-blocking film for blocking light from a light source such as a backlight. For this reason, the oxide semiconductor film 808 included in the transistor is not irradiated with light, so that a variation in the electrical characteristics of the transistor can be suppressed.

The conductive film 821 b is electrically connected to an oxide semiconductor film 819 a functioning as a pixel electrode. A common electrode 829 is provided over the oxide semiconductor film 819 a with an insulating film (not shown in FIG. 28) positioned therebetween.

The common electrode 829 includes stripe regions extending in a direction intersecting with the signal line. The stripe regions are connected to a region extending in a direction parallel or substantially parallel to the signal line. Accordingly, the stripe regions of the common electrode 829 are at the same potential in a plurality of pixels included in the liquid crystal display device 880.

The capacitor 855 is formed in a region where the oxide semiconductor film 819 a and the common electrode 829 overlap with each other. The oxide semiconductor film 819 a and the common electrode 829 each have a light-transmitting property. That is, the capacitor 855 has a light-transmitting property.

Owing to the light-transmitting property of the capacitor 855, the capacitor 855 can be formed large (in a large area) in the pixel 870. Thus, a display device with a large amount of charge capacity as well as an aperture ratio increased to typically 50% or more, preferably 60% or more can be provided. For example, in a high-resolution display device such as a liquid crystal display device, the area of a pixel is small and accordingly the area of a capacitor is also small. For this reason, the amount of charge stored in the capacitor is small in the high-resolution display device. However, since the capacitor 855 of this embodiment transmits light, when it is provided in a pixel, a sufficient amount of charge can be obtained in the pixel and the aperture ratio can be improved. Typically, the capacitor 855 can be favorably used for a high-resolution display device with a pixel density of 200 ppi or more, 300 ppi or more, or furthermore, 500 ppi or more.

In a liquid crystal display device, as the capacitance value of a capacitor is increased, a period during which the alignment of liquid crystal molecules of a liquid crystal element can be kept constant in the state where an electric field is applied can be made longer. When the period can be made longer in a display device which displays a still image, the number of times of rewriting image data can be reduced, leading to a reduction in power consumption. Further, according to the structure of this embodiment, the aperture ratio can be improved even in a high-resolution display device, which makes it possible to use light from a light source such as a backlight efficiently, so that power consumption of the display device can be reduced.

FIG. 29 is a cross-sectional view taken along the dashed-dotted line Q1-R1 and the dashed-dotted line S1-T1 in FIG. 28. The transistor 852 illustrated in FIG. 29 is a channel-etched transistor. Note that the transistor 852 in the channel length direction and the capacitor 855 are illustrated in the cross-sectional view taken along the dashed-dotted line Q1-R1, and the transistor 852 in the channel width direction is illustrated in the cross-sectional view taken along dashed-dotted line S1-T1. Note that the oxide semiconductor film 819 b functioning as a second gate electrode is not illustrated in FIG. 28 for simplicity of the drawing.

The transistor 852 shown in FIG. 29 includes the conductive film 813 functioning as a first gate electrode over a substrate 811, an insulating film 815 formed over the substrate 811 and the conductive film 813 functioning as a first gate electrode, an insulating film 817 formed over the insulating film 815, the oxide semiconductor film 808 overlapping the conductive film 813 functioning as a gate electrode with the insulating film 815 and the insulating film 817 positioned therebetween, the conductive films 821 a and 821 b functioning as a source electrode and a drain electrode and in contact with the oxide semiconductor film 808, insulating films 823 and 825 provided over the oxide semiconductor film 808 and the conductive films 821 a and 821 b functioning as a source electrode and a drain electrode, the oxide semiconductor film 819 b functioning as a second gate electrode over the insulating film 825, and an insulating film 827 over the insulating film 825 and the oxide semiconductor film 819 b.

The oxide semiconductor film 819 a is formed over the insulating film 825. The oxide semiconductor film 819 a is electrically connected to one of the conductive films 821 a and 821 b functioning as a source electrode and a drain electrode (here, the conductive film 821 b) through an opening portion provided in the insulating film 823 and the insulating film 825. The insulating film 827 is formed over the insulating film 825 and the oxide semiconductor film 819 a. The common electrode 829 is formed over the insulating film 827.

A region where the oxide semiconductor film 819 a, the insulating film 827, and the common electrode 829 overlap with one another functions as the capacitor 855.

Note that a cross-sectional structure of one embodiment of the present invention is not limited thereto. For example, the oxide semiconductor film 819 a may have a slit. Alternatively, the oxide semiconductor film 819 a may have a comb-like shape.

Note that as shown in FIG. 30, the common electrode 829 may be provided over the insulating film 828 provided over the insulating film 827. The insulating film 828 functions as a planarization film.

3-4. Structure Example 2 of Horizontal Electric Field Mode Element Substrate

FIG. 31 is a top view of a plurality of pixels 870 d, 870 e, and 870 f included in the liquid crystal display device 880. The structures of the pixels 870 d, 870 e, and 870 f are different from those of the pixels shown in FIG. 28. The liquid crystal display device 880 shown in FIG. 31 is driven in the IPS mode.

In FIG. 31, the conductive film 813 functioning as a scan line extends in the horizontal direction in the drawing. The conductive film 821 a functioning as a signal line extends substantially perpendicularly to the scan line (in the vertical direction in the drawing) so as to have a partly bent dogleg shape (V-like shape). As the conductive film 813 functioning as a scan line is electrically connected to the gate driver 874, and the conductive film 821 a functioning as a signal line is electrically connected to the source driver 876 (see FIG. 27A).

The transistor 852 is provided in the vicinity of the intersection portion of the scan line and the signal line. The transistor 852 includes the conductive film 813 functioning as a gate electrode; the gate insulating film (not illustrated in FIG. 31); an oxide semiconductor film 808 where a channel region is formed, over the gate insulating film; and the conductive films 821 a and 821 b functioning as a source electrode and a drain electrode. The conductive film 813 also functions as a scan line, and a region of the conductive film 813 that overlaps with the oxide semiconductor film 808 functions as the gate electrode of the transistor 852. The conductive film 821 a serves as a signal line, and the conductive film 821 a that overlaps with the oxide semiconductor film 808 serves as the source electrode of the transistor 852. Further, in the top view of FIG. 31, an end portion of the scan line is located on the outer side than an end portion of the oxide semiconductor film 808. Thus, the scan line functions as a light-blocking film for blocking light from a light source such as a backlight. For this reason, the oxide semiconductor film 808 included in the transistor is not irradiated with light, so that a variation in the electrical characteristics of the transistor can be suppressed.

The conductive film 821 b is electrically connected to the oxide semiconductor film 819 a functioning as a pixel electrode. The oxide semiconductor film 819 a has a comb-like shape. An insulating film (not illustrated in FIG. 31) is provided over the oxide semiconductor film 819 a, and the common electrode 829 is provided over the insulating film. The common electrode 829 has a comb-like shape so as to overlap a part of the oxide semiconductor film 819 a and mesh with the oxide semiconductor film 819 a in the top view. The common electrode 829 is connected to a region extending in a direction parallel or substantially parallel to the scan line. Accordingly, the common electrodes 829 are at the same potential in pixels included in the liquid crystal display device 880. Note that the oxide semiconductor film 819 a and the common electrode 829 have a dogleg shape bent along the signal line (the conductive film 821 a).

The capacitor 855 is formed in a region where the oxide semiconductor film 819 a and the common electrode 829 overlap with each other. The oxide semiconductor film 819 a and the common electrode 829 each have a light-transmitting property. That is, the capacitor 855 has a light-transmitting property.

FIG. 32 is a cross-sectional view taken along the dashed-dotted line Q2-R2 and the dashed-dotted line S2-T2 in FIG. 31. The transistor 852 illustrated in FIG. 32 is a channel-etched transistor. Note that the transistor 852 in the channel length direction and the capacitor 855 are illustrated in the cross-sectional view taken along the dashed-dotted line Q2-R2, and the transistor 852 in the channel width direction is illustrated in the cross-sectional view taken along dashed-dotted line S2-T2. Note that the oxide semiconductor film 819 b functioning as a second gate electrode is not illustrated in FIG. 31 for simplicity of the drawing.

The transistor 852 shown in FIG. 32 includes the conductive film 813 functioning as a gate electrode over a substrate 811, an insulating film 815 formed over the substrate 811 and the conductive film 813 functioning as a gate electrode, an insulating film 817 formed over the insulating film 815, the oxide semiconductor film 808 overlapping the conductive film 813 functioning as a gate electrode with the insulating film 815 and the insulating film 817 positioned therebetween, the conductive films 821 a and 821 b functioning as a source electrode and a drain electrode and in contact with the oxide semiconductor film 808, insulating films 823 and 825 provided over the oxide semiconductor film 808 and the conductive films 821 a and 821 b functioning as a source electrode and a drain electrode, the oxide semiconductor film 819 b functioning as a second gate electrode over the insulating film 825, and an insulating film 827 over the insulating film 825 and the oxide semiconductor film 819 b.

The oxide semiconductor film 819 a is formed over the insulating film 825. The oxide semiconductor film 819 a is electrically connected to one of the conductive films 821 a and 821 b functioning as a source electrode and a drain electrode (here, the conductive film 821 b) through an opening portion provided in the insulating film 823 and the insulating film 825. The insulating film 827 is formed over the insulating film 825 and the oxide semiconductor film 819 a. The common electrode 829 is formed over the insulating film 827.

A region where the oxide semiconductor film 819 a, the insulating film 827, and the common electrode 829 overlap with one another functions as the capacitor 855.

In the liquid crystal display device shown in FIG. 31 and FIG. 32, the capacitor included in the pixel has a structure in which an end portion and the vicinity of the end portion of the oxide semiconductor film 819 a overlap with an end portion and the vicinity of the end portion of the common electrode 829. The capacitor having such a structure can be of appropriate size which is not too large in a large liquid crystal display device.

Note that as shown in FIG. 33, the common electrode 829 may be provided over the insulating film 828 provided over the insulating film 827.

A structure in which the oxide semiconductor film 819 a does not overlap with the common electrode 829 as shown in FIG. 34 and FIG. 35 may be used. The positional relation of the oxide semiconductor film 819 a and the common electrode 829 can be determined as appropriate depending on the size of a capacitor in accordance with the driving method and the resolution of the display device. Note that the common electrode 829 included in the display device shown in FIG. 35 may be provided over the insulating film 828 functioning as a planarization film (see FIG. 36).

In the liquid crystal display device shown in FIG. 31 and FIG. 32, a width (d1 in FIG. 32) of a region extending in a direction parallel or substantially parallel to the signal line (the conductive film 821 a) in the oxide semiconductor film 819 a is smaller than a width (d2 in FIG. 32) of a region extending in a direction parallel or substantially parallel to the signal line in the common electrode 829; however, the liquid crystal display device is not limited to this structure. As shown in FIG. 37 and FIG. 38, the width d1 may be larger than the width d2. The width d1 may be equal to the width d2. In one pixel (e.g., the pixel 870 d), widths of a plurality of regions extending in a direction parallel or substantially parallel to the signal line in the oxide semiconductor film 819 a and/or the common electrode 829 may be different from each other.

A structure as shown in FIG. 39 may be used in which, in the insulating film 828 over the insulating film 827, only a region overlapping with the common electrode 829 over the insulating film 828 is left and the other region is removed. In this case, the insulating film 828 can be etched using the common electrode 829 as a mask. The unevenness of the common electrode 829 over the insulating film 828 functioning as a planarization film can be suppressed, and the insulating film 828 has a gently-sloping side surface in a region from an end portion of the common electrode 829 toward the insulating film 827. Alternatively, a structure as shown in FIG. 40 may be used in which, in a surface of the insulating film 828, part of a region parallel to the substrate 811 is not covered with the common electrode 829.

As shown in FIG. 41 and FIG. 42, a common electrode may be provided over a film over which the oxide semiconductor film 819 a is formed, i.e., over the insulating film 825. A common electrode 819 c shown in FIG. 41 and FIG. 42 can be formed using a material used for forming the oxide semiconductor film 819 a and by processing an oxide semiconductor film used for forming the oxide semiconductor film 819 a.

3-5. Vertical Alignment Mode Liquid Crystal Display Device

A structure of a pixel including a liquid crystal element which operates in a vertical alignment (VA) mode is described with reference to FIG. 43 and FIG. 44. FIG. 43 is a top view of the pixel included in the liquid crystal display device. FIG. 44 is a side view including a cross section taken along the line A1-B1 in FIG. 43. FIG. 45 is an equivalent circuit diagram of the pixel included in the liquid crystal display device.

A vertical alignment (VA) is a mode for controlling alignment of liquid crystal molecules of a liquid crystal display panel. In the VA liquid crystal display device, liquid crystal molecules are aligned in a vertical direction with respect to a panel surface when no voltage is applied.

In the following description, it is devised to particularly separate pixels into some regions (sub-pixels) so that molecules are aligned in different directions in the respective regions. This is referred to as multi-domain or multi-domain design. In the following description, a liquid crystal display device with multi-domain design is described.

In FIG. 43, Z1 is a top view of a substrate 600 provided with a pixel electrode 624. Z3 is a top view of a substrate 601 provided with a common electrode 640. Z2 is a top view illustrating a state where the substrate 601 provided with the common electrode 640 is overlapped with the substrate 600 provided with the pixel electrode 624.

The transistor 628, the pixel electrode 624 connected thereto, and the capacitor 630 are formed over the substrate 600. A drain electrode 618 of the transistor 628 is electrically connected to the pixel electrode 624 through an opening 633 provided in an insulating film 623 and an insulating film 625. An insulating film 627 is provided over the pixel electrode 624.

The transistor described in Embodiment 1 can be used as the transistor 628.

The capacitor 630 includes a wiring 613 over a capacitor wiring 604 as a first capacitor wiring, the insulating film 623, the insulating film 625, and the pixel electrode 624. The capacitor wiring 604 can be formed concurrently with the gate wiring 615 of the transistor 628. The wiring 613, the drain electrode 618, and the wiring 616 can be formed of the same material at the same time.

The oxide semiconductor film with a low sheet resistivity described in Embodiment 1 can be used for the pixel electrode 624.

The pixel electrode 624 is provided with a slit 646. The slit 646 is provided for controlling the alignment of the liquid crystals.

A transistor 629, a pixel electrode 626 connected thereto, and a capacitor 631 can be formed in manners similar to those for the transistor 628, the pixel electrode 624, and the capacitor 630, respectively. Both the transistors 628 and 629 are connected to the wiring 616. The wiring 616 functions as a source electrode in each of the transistor 628 and the transistor 629. A pixel of the liquid crystal display panel described in this embodiment includes the pixel electrodes 624 and 626. Each of the pixel electrodes 624 and 626 is a sub-pixel.

The substrate 601 is provided with a coloring film 636 and the common electrode 640, and the common electrode 640 is provided with a structure body 644. The common electrode 640 is provided with a slit 647. An alignment film 648 is formed over the pixel electrode 624. Similarly, an alignment film 645 is formed on the common electrode 640 and the structure body 644. A liquid crystal layer 650 is formed between the substrate 600 and the substrate 601.

The slit 647 formed in the common electrode 640 and the structure body 644 each have a function of controlling the alignment of liquid crystals.

When a voltage is applied to the pixel electrode 624 provided with the slit 646, a distorted electric field (an oblique electric field) is generated in the vicinity of the slit 646. The slit 646 and the structure body 644 on the substrate 601 side and the slit 647 are alternately arranged in an engaging manner, and thus, an oblique electric field is effectively generated to control alignment of the liquid crystal, so that a direction of alignment of the liquid crystal varies depending on location. That is, the viewing angle of a liquid crystal display panel is increased by employing multi-domain. Note that one of the structure body 644 and the slit 647 may be provided for the substrate 601.

FIG. 44 illustrates a state where the substrate 600 and the substrate 601 are overlapped with each other and liquid crystal is injected therebetween. A liquid crystal element is formed by overlapping of the pixel electrode 624, the liquid crystal layer 650, and the common electrode 640.

FIG. 45 illustrates an equivalent circuit of this pixel structure. Both the transistors 628 and 629 are connected to the gate wiring 602 and the wiring 616. In this case, by making the potential of the capacitor wiring 604 different from that of a capacitor wiring 605, operation of a liquid crystal element 651 can be different from that of a liquid crystal element 652. In other words, each potential of the capacitor wirings 604 and 605 is individually controlled, whereby orientation of liquid crystals is precisely controlled to expand a viewing angle.

This embodiment can be combined with any of the other embodiments in this specification as appropriate.

Embodiment 4

In this embodiment, a display device that includes a semiconductor device of one embodiment of the present invention is described with reference to FIGS. 46A and 46B and FIGS. 47A and 47B. Note that in this embodiment, a structure in which an electroluminescence (EL) element is included as a display element of the display device is specifically described.

4-1. Display Device

The display device illustrated in FIG. 46A includes a region including pixels of display elements (hereinafter the region is referred to as a pixel portion 502), a circuit portion provided outside the pixel portion 502 and including a circuit for driving the pixels (hereinafter the portion is referred to as a driver circuit portion 504), circuits each having a function of protecting an element (hereinafter the circuits are referred to as protection circuits 506), and a terminal portion 507. Note that the protection circuits 506 are not necessarily provided.

A part or the whole of the driver circuit portion 504 is preferably formed over a substrate over which the pixel portion 502 is formed, in which case the number of components and the number of terminals can be reduced. When a part or the whole of the driver circuit portion 504 is not formed over the substrate over which the pixel portion 502 is formed, the part or the whole of the driver circuit portion 504 can be mounted by COG or tape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for driving display elements arranged in X rows (X is a natural number of 2 or more) and Y columns (Y is a natural number of 2 or more) (hereinafter, such circuits are referred to as pixel circuits 501). The driver circuit portion 504 includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (hereinafter, the circuit is referred to as a gate driver 504 a) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (hereinafter, the circuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gate driver 504 a receives a signal for driving the shift register through the terminal portion 507 and outputs a signal. For example, the gate driver 504 a receives a start pulse signal, a clock signal, or the like and outputs a pulse signal. The gate driver 504 a has a function of controlling the potentials of wirings supplied with scan signals (hereinafter, such wirings are referred to as scan lines GL_1 to GL_X). Note that a plurality of gate drivers 504 a may be provided to control the scan lines GL_1 to GL_X separately. Alternatively, the gate driver 504 a has a function of supplying an initialization signal. Without being limited thereto, the gate driver 504 a can supply another signal. For example, as shown in FIG. 46A, the gate driver 504 a is electrically connected to wirings for controlling the potential of the light-emitting element (hereinafter such wirings are referred to as ANODE_1 to ANODE_X).

The source driver 504 b includes a shift register or the like. The source driver 504 b receives a signal (image signal) from which a data signal is derived, as well as a signal for driving the shift register, through the terminal portion 507. The source driver 504 b has a function of generating a data signal to be written to the pixel circuit 501 which is based on the image signal. In addition, the source driver 504 b has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like. Furthermore, the source driver 504 b has a function of controlling the potentials of wirings supplied with data signals (hereinafter such wirings are referred to as data lines DL_1 to DL_Y). Alternatively, the source driver 504 b has a function of supplying an initialization signal. Without being limited thereto, the source driver 504 b can supply another signal.

The source driver 504 b includes a plurality of analog switches or the like, for example. The source driver 504 b can output, as the data signals, signals obtained by time-dividing the image signal by sequentially turning on the plurality of analog switches. The source driver 504 b may include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality of pixel circuits 501 through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively. Writing and holding of the data signal to and in each of the plurality of pixel circuits 501 are controlled by the gate driver 504 a. For example, to the pixel circuit 501 in the m-th row and the n-th column (m is a natural number of less than or equal to X, and n is a natural number of less than or equal to 1), a pulse signal is input from the gate driver 504 a through the scan line GL_m, and a data signal is input from the source driver 504 b through the data line DL_n in accordance with the potential of the scan line GL_m.

The protection circuit 506 shown in FIG. 46A is connected to, for example, the scan line GL between the gate driver 504 a and the pixel circuit 501. Alternatively, the protection circuit 506 is connected to the data line DL between the source driver 504 b and the pixel circuit 501. Alternatively, the protection circuit 506 can be connected to a wiring between the gate driver 504 a and the terminal portion 507. Alternatively, the protection circuit 506 can be connected to a wiring between the source driver 504 b and the terminal portion 507. Note that the terminal portion 507 means a portion having terminals for inputting power, control signals, and image signals to the display device from external circuits.

The protection circuit 506 is a circuit that electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is applied to the wiring connected to the protection circuit.

As illustrated in FIG. 46A, the protection circuits 506 are provided for the pixel portion 502 and the driver circuit portion 504, so that the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like can be improved. Note that the configuration of the protection circuits 506 is not limited to that, and for example, the protection circuit 506 may be configured to be connected to the gate driver 504 a or the protection circuit 506 may be configured to be connected to the source driver 504 b. Alternatively, the protection circuit 506 may be configured to be connected to the terminal portion 507.

In FIG. 46A, an example in which the driver circuit portion 504 includes the gate driver 504 a and the source driver 504 b is shown; however, the structure is not limited thereto. For example, only the gate driver 504 a may be formed and a separately prepared substrate where a source driver circuit is formed (e.g., a driver circuit substrate formed with a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted.

4-2. Structure Example of Pixel Circuit

Each of the plurality of pixel circuits 501 in FIG. 46A can have the structure illustrated in FIG. 46B, for example.

The pixel circuit 501 illustrated in FIG. 46B includes transistors 552 and 554, a capacitor 562, and a light-emitting element 572. Any of the transistors described in the above embodiment can be used as one or both of the transistors 552 and 554.

One of a source electrode and a drain electrode of the transistor 552 is electrically connected to a wiring to which a data signal is supplied (hereinafter referred to as a signal line DL_n). A gate electrode of the transistor 552 is electrically connected to a wiring to which a gate signal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 552 has a function of controlling whether to write a data signal by being turned on or off.

One of a pair of electrodes of the capacitor 562 is electrically connected to the other of the source electrode and the drain electrode of the transistor 552. The other of the pair of electrodes of the capacitor 562 is electrically connected to a second gate electrode (also referred to as a back gate electrode) of the transistor 554. The capacitor 562 functions as a storage capacitor for storing written data.

One of a source electrode and a drain electrode of the transistor 554 is electrically connected to an anode line (ANODE_m).

One of an anode and a cathode of the light-emitting element 572 is electrically connected to the other of the source electrode and the drain electrode of the transistor 554, and the other of the anode and the cathode of the light-emitting element 572 is electrically connected to a cathode line (CATHODE). Note that the other of the pair of electrodes of the capacitor 562 is electrically connected to the one of the anode and the cathode of the light-emitting element 572.

As the light-emitting element 572, for example, an organic EL element can be used. Note that the light-emitting element 572 is not limited to an organic EL element; an inorganic EL element including an inorganic material may be used.

For example, in the display device including the pixel circuit 501 in FIG. 46B, the pixel circuits 501 are sequentially selected row by row by the gate driver 504 a illustrated in FIG. 46A, whereby the transistors 552 are turned on and a data signal is written.

When the transistors 552 are turned off, the pixel circuits 501 in which the data has been written are brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor 554 is controlled in accordance with the potential of the written data signal. The light-emitting element 572 emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image can be displayed.

Although the structure including the light-emitting element 572 as a display element of the display device is described in this embodiment, one embodiment of the present invention is not limited to the structure and a variety of elements may be included in the display device.

For example, the display device includes at least one of the liquid crystal element, an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor (a transistor that emits light depending on current), an electron emitter, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma display panel (PDP), a display element using micro electro mechanical systems (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), MIRASOL (registered trademark), an interferometric modulator display (IMOD) element, a MEMS shutter display element, an optical-interference-type MEMS display element, an electrowetting element, a piezoelectric ceramic display, a display element using a carbon nanotube, and the like. Alternatively, the display device may include a display medium whose contrast, luminance, reflectivity, transmittance, or the like is changed by electrical or magnetic effect. Examples of display devices including electron emitters are a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Examples of display devices including liquid crystal elements include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). An example of a display device including electronic ink or electrophoretic elements is electronic paper. In the case of a transflective liquid crystal display or a reflective liquid crystal display, some of or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes are formed to contain aluminum, silver, or the like. In such a case, a memory circuit such as an SRAM can be provided under the reflective electrodes. Thus, the power consumption can be further reduced.

A progressive type display, an interlace type display, or the like can be employed as the display type of the display device. Further, color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, four pixels of the R pixel, the G pixel, the B pixel, and a W (white) pixel may be included. Alternatively, a color element may be composed of two colors among R, G, and B as in PenTile layout. The two colors may differ among color elements. Alternatively, one or more colors of yellow, cyan, magenta, and the like may be added to RGB. Further, the size of a display region may be different depending on respective dots of the color elements. Embodiments of the disclosed invention are not limited to a display device for color display; the disclosed invention can also be applied to a display device for monochrome display.

White light (W) may be emitted from a backlight (e.g., an organic EL element, an inorganic EL element, an LED, or a fluorescent lamp) in the display device. Furthermore, a coloring layer (also referred to as a color filter) may be provided in the display device. As the coloring layer, red (R), green (G), blue (B), yellow (Y), or the like may be combined as appropriate, for example. With the use of the coloring layer, higher color reproducibility can be obtained than in the case without the coloring layer. In this case, by providing a region with the coloring layer and a region without the coloring layer, white light in the region without the coloring layer may be directly utilized for display. By partly providing the region without the coloring layer, a decrease in luminance due to the coloring layer can be suppressed, and 20% to 30% of power consumption can be reduced in some cases when an image is displayed brightly. Note that in the case where full-color display is performed using self-luminous elements such as organic EL elements or inorganic EL elements, the elements may emit light of their respective colors R, G, B, Y, and W. By using self-luminous elements, power consumption can be further reduced as compared to the case of using the coloring layer in some cases.

4-3. Structure Example of Pixel of Display Device

An example of a display device including the pixel circuit shown in FIG. 46B is described with reference to FIGS. 47A and 47B. FIG. 47A is a top view of a pixel portion of the display device, and FIG. 47B is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 47A. Note that in FIG. 47A, some components are not illustrated in order to avoid complexity of the drawing.

The display device shown in FIGS. 47A and 47B includes a conductive film 704 functioning as a first gate electrode over a substrate 702, insulating films 706 and 707 over the conductive film 704, an oxide semiconductor film 708 over the insulating film 707, conductive films 712 a and 712 b functioning as a source electrode and a drain electrode over the insulating film 707 and the oxide semiconductor film 708, a conductive film 712 c over the insulating film 707, insulating films 714 and 716 covering the oxide semiconductor film 708 and the conductive films 712 a, 712 b, and 712 c, an oxide semiconductor film 720 functioning as a second gate electrode over the insulating film 716, an insulating film 718 over the insulating film 716 and the oxide semiconductor film 720, an insulating film 722 functioning as a planarization insulating film over the insulating film 718, conductive films 724 a and 724 b functioning as pixel electrodes over the insulating film 722, a structure body 726 having a function of suppressing electrical connection between the conductive film 724 a and the conductive film 724 b, an EL layer 728 over the conductive films 724 a and 724 b and the structure body 726, and a conductive film 730 over the EL layer 728.

The conductive film 712 c is electrically connected to the conductive film 704 through an opening portion 752 c provided in the insulating films 706 and 707. The oxide semiconductor film 720 functioning as a second gate electrode is electrically connected to the conductive film 712 b through an opening portion 752 a provided in the insulating films 714 and 716. The conductive film 724 a is electrically connected to the conductive film 712 b through an opening portion 752 b provided in the insulating films 714, 716, 718, and 722.

The conductive film 724 a functioning as a pixel electrode, the EL layer 728, and the conductive film 730 form the light-emitting element 572. Note that the EL layer 728 can be formed by any of the following methods: a sputtering method, an evaporation method (including a vacuum evaporation method), a printing method (such as relief printing, intaglio printing, gravure printing, planography printing, and stencil printing), an ink jet method, a coating method, and the like.

When a structure including two transistors and one capacitor as shown in FIG. 46B and FIGS. 47A and 47B is used as a structure of a pixel of the display device, the number of wirings can be reduced. For example, the pixel shown in FIG. 46B and FIG. 47A includes three wirings, i.e., a gate line, a data line, and the anode line. A pixel with such a structure in the display device can have a high aperture ratio. Moreover, by reducing the number of wirings, a short circuit between adjacent wirings is unlikely to occur, for example. Thus, display devices can be provided with high yield.

The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments.

Embodiment 5

In this embodiment, a display device including the semiconductor device of one embodiment of the present invention and an electronic device in which the display device is provided with an input device will be described with reference to FIGS. 48A and 48B, FIGS. 49A and 49B, FIG. 50, FIGS. 51A and 51B, FIGS. 52A and 52B, and FIG. 53.

<5-1. Touch Panel>

In this embodiment, a touch panel 2000 including a display device and an input device will be described as an example of an electronic device. In addition, an example in which a touch sensor is used as an input device will be described.

FIGS. 48A and 48B are perspective views of the touch panel 2000. Note that FIGS. 48A and 48B illustrate only main components of the touch panel 2000 for simplicity.

The touch panel 2000 includes a display device 2501 and a touch sensor 2595 (see FIG. 48B). The touch panel 2000 also includes a substrate 2510, a substrate 2570, and a substrate 2590. The substrate 2510, the substrate 2570, and the substrate 2590 each have flexibility. Note that one or all of the substrates 2510, 2570, and 2590 may be inflexible.

The display device 2501 includes a plurality of pixels over the substrate 2510 and a plurality of wirings 2511 through which signals are supplied to the pixels. The plurality of wirings 2511 are led to a peripheral portion of the substrate 2510, and parts of the plurality of wirings 2511 form a terminal 2519. The terminal 2519 is electrically connected to an FPC 2509(1).

The substrate 2590 includes the touch sensor 2595 and a plurality of wirings 2598 electrically connected to the touch sensor 2595. The plurality of wirings 2598 are led to a peripheral portion of the substrate 2590, and parts of the plurality of wirings 2598 form a terminal. The terminal is electrically connected to an FPC 2509(2). Note that in FIG. 48B, electrodes, wirings, and the like of the touch sensor 2595 provided on the back side of the substrate 2590 (the side facing the substrate 2510) are indicated by solid lines for clarity.

As the touch sensor 2595, a capacitive touch sensor can be used. Examples of the capacitive touch sensor are a surface capacitive touch sensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor are a self capacitive touch sensor and a mutual capacitive touch sensor, which differ mainly in the driving method. The use of a mutual capacitive type is preferable because multiple points can be sensed simultaneously.

Note that the touch sensor 2595 illustrated in FIG. 48B is an example of using a projected capacitive touch sensor.

Note that a variety of sensors that can sense proximity or touch of a sensing target such as a finger can be used as the touch sensor 2595.

The projected capacitive touch sensor 2595 includes electrodes 2591 and electrodes 2592. The electrodes 2591 are electrically connected to any of the plurality of wirings 2598, and the electrodes 2592 are electrically connected to any of the other wirings 2598.

The electrodes 2592 each have a shape of a plurality of quadrangles arranged in one direction with one corner of a quadrangle connected to one corner of another quadrangle as illustrated in FIGS. 48A and 48B.

The electrodes 2591 each have a quadrangular shape and are arranged in a direction intersecting with the direction in which the electrodes 2592 extend.

A wiring 2594 electrically connects two electrodes 2591 between which the electrode 2592 is positioned. The intersecting area of the electrode 2592 and the wiring 2594 is preferably as small as possible. Such a structure allows a reduction in the area of a region where the electrodes are not provided, reducing variation in transmittance. As a result, variation in luminance of light passing through the touch sensor 2595 can be reduced.

Note that the shapes of the electrodes 2591 and the electrodes 2592 are not limited thereto and can be any of a variety of shapes. For example, a structure may be employed in which the plurality of electrodes 2591 are arranged so that gaps between the electrodes 2591 are reduced as much as possible, and the electrodes 2592 are spaced apart from the electrodes 2591 with an insulating layer interposed therebetween to have regions not overlapping with the electrodes 2591. In this case, it is preferable to provide, between two adjacent electrodes 2592, a dummy electrode electrically insulated from these electrodes because the area of regions having different transmittances can be reduced.

Note that as a material of the conductive films such as the electrodes 2591, the electrodes 2592, and the wirings 2598, that is, wirings and electrodes forming the touch panel, a transparent conductive film containing indium oxide, tin oxide, zinc oxide, or the like (e.g., ITO) can be given. For example, a low-resistance material is preferably used as a material that can be used as the wirings and electrodes forming the touch panel. For example, silver, copper, aluminum, a carbon nanotube, graphene, or a metal halide (such as a silver halide) may be used. Alternatively, a metal nanowire including a plurality of conductors with an extremely small width (for example, a diameter of several nanometers) may be used. Further alternatively, a net-like metal mesh with a conductor may be used. For example, an Ag nanowire, a Cu nanowire, an Al nanowire, an Ag mesh, a Cu mesh, or an Al mesh may be used. For example, in the case of using an Ag nanowire as the wirings and electrodes forming the touch panel, a visible light transmittance of 89% or more and a sheet resistance of 40 Ω/cm² or more and 100 Ω/cm² or less can be achieved. Since the above-described metal nanowire, metal mesh, carbon nanotube, graphene, and the like, which are examples of the material that can be used as the wirings and electrodes forming the touch panel, have high visible light transmittances, they may be used as electrodes of display elements (e.g., a pixel electrode or a common electrode).

<5-2. Display Device>

Next, the display device 2501 will be described in detail with reference to FIGS. 49A and 49B. FIGS. 49A and 49B correspond to cross-sectional views taken along dashed-dotted line X1-X2 in FIG. 48B.

The display device 2501 includes a plurality of pixels arranged in a matrix. Each of the pixels includes a display element and a pixel circuit for driving the display element.

[Structure with EL Element as Display Element]

First, a structure that uses an EL element as a display element will be described below with reference to FIG. 49A. In the following description, an example of using an EL element that emits white light will be described; however, the EL element is not limited to this element. For example, EL elements that emit light of different colors may be included so that the light of different colors can be emitted from adjacent pixels.

For the substrate 2510 and the substrate 2570, for example, a flexible material with a vapor permeability of lower than or equal to 10⁻⁵ g/(m²·day), preferably lower than or equal to 10′ g/(m²·day) can be favorably used. Alternatively, materials whose thermal expansion coefficients are substantially equal to each other are preferably used for the substrate 2510 and the substrate 2570. For example, the coefficients of linear expansion of the materials are preferably lower than or equal to 1×10⁻³/K, further preferably lower than or equal to 5×10⁻⁵/K, and still further preferably lower than or equal to 1×10⁻⁵/K.

Note that the substrate 2510 is a stacked body including an insulating layer 2510 a for preventing impurity diffusion into the EL element, a flexible substrate 2510 b, and an adhesive layer 2510 c for attaching the insulating layer 2510 a and the flexible substrate 2510 b to each other. The substrate 2570 is a stacked body including an insulating layer 2570 a for preventing impurity diffusion into the EL element, a flexible substrate 2570 b, and an adhesive layer 2570 c for attaching the insulating layer 2570 a and the flexible substrate 2570 b to each other.

For the adhesive layer 2510 c and the adhesive layer 2570 c, for example, materials that include polyester, polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate, an acrylic resin, polyurethane, an epoxy resin, or a resin having a siloxane bond can be used.

A sealing layer 2560 is provided between the substrate 2510 and the substrate 2570. The sealing layer 2560 preferably has a refractive index higher than that of air. In the case where light is extracted to the sealing layer 2560 side as illustrated in FIG. 49A, the sealing layer 2560 can also serve as an optical element.

A sealant may be formed in the peripheral portion of the sealing layer 2560. With the use of the sealant, an EL element 2550 can be provided in a region surrounded by the substrate 2510, the substrate 2570, the sealing layer 2560, and the sealant. Note that an inert gas (such as nitrogen or argon) may be used instead of the sealing layer 2560. A drying agent may be provided in the inert gas so as to adsorb moisture or the like. For example, an epoxy-based resin or a glass frit is preferably used as the sealant. As a material used for the sealant, a material which is impermeable to moisture or oxygen is preferably used.

The display device 2501 illustrated in FIG. 49A includes a pixel 2505. The pixel 2505 includes a light-emitting module 2580, the EL element 2550 and a transistor 2502 t that can supply electric power to the EL element 2550. Note that the transistor 2502 t functions as part of the pixel circuit.

The light-emitting module 2580 includes the EL element 2550 and a coloring layer 2567. The EL element 2550 includes a lower electrode, an upper electrode, and an EL layer between the lower electrode and the upper electrode.

In the case where the sealing layer 2560 is provided on the light extraction side, the sealing layer 2560 is in contact with the EL element 2550 and the coloring layer 2567.

The coloring layer 2567 is positioned in a region overlapping with the EL element 2550. Accordingly, part of light emitted from the EL element 2550 passes through the coloring layer 2567 and is emitted to the outside of the light-emitting module 2580 as indicated by an arrow in FIG. 49A.

The display device 2501 includes a light-blocking layer 2568 on the light extraction side. The light-blocking layer 2568 is provided so as to surround the coloring layer 2567.

The coloring layer 2567 is a coloring layer having a function of transmitting light in a particular wavelength region. For example, a color filter for transmitting light in a red wavelength range, a color filter for transmitting light in a green wavelength range, a color filter for transmitting light in a blue wavelength range, a color filter for transmitting light in a yellow wavelength range, or the like can be used. Each color filter can be formed with any of various materials by a printing method, an inkjet method, an etching method using a photolithography technique, or the like.

An insulating layer 2521 is provided in the display device 2501. The insulating layer 2521 covers the transistor 2502 t and the like. Note that the insulating layer 2521 has a function of covering the roughness caused by the pixel circuit to provide a flat surface. The insulating layer 2521 may have a function of suppressing impurity diffusion. This can prevent the reliability of the transistor 2502 t or the like from being lowered by impurity diffusion.

The EL element 2550 is formed over the insulating layer 2521. A partition 2528 is provided so as to overlap with an end portion of the lower electrode of the EL element 2550. Note that a spacer for controlling the distance between the substrate 2510 and the substrate 2570 may be formed over the partition 2528.

A scan line driver circuit 2504 includes a transistor 2503 t and a capacitor 2503 c. Note that the driver circuit can be formed in the same process and over the same substrate as those of the pixel circuits.

The wirings 2511 through which signals can be supplied are provided over the substrate 2510. The terminal 2519 is provided over the wirings 2511. The FPC 2509(1) is electrically connected to the terminal 2519. The FPC 2509(1) has a function of supplying a video signal, a clock signal, a start signal, a reset signal, or the like. Note that the FPC 2509(1) may be provided with a printed wiring board (PWB).

Any of the transistors described in the above embodiments may be used as one or both of the transistors 2502 t and 2503 t. The transistors used in this embodiment each include an oxide semiconductor film which is highly purified and has high crystallinity. In the transistors, the current in an off state (off-state current) can be made small. Accordingly, an electrical signal such as an image signal can be held for a longer period, and a writing interval can be set longer in an on state. Accordingly, the frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption. Note that the detail of the refresh operation will be described later.

In addition, the transistors used in this embodiment can have relatively high field-effect mobility and thus are capable of high speed operation. For example, with such transistors which can operate at high speed used for the display device 2501, a switching transistor of a pixel circuit and a driver transistor in a driver circuit portion can be formed over one substrate. That is, a semiconductor device formed using a silicon wafer or the like is not additionally needed as a driver circuit, by which the number of components of the semiconductor device can be reduced. In addition, by using a transistor which can operate at high speed in a pixel circuit, a high-quality image can be provided.

[Structure with Liquid Crystal Element as Display Element]

Next, a structure including a liquid crystal element as a display element is described below with reference to FIG. 49B. In the description below, a reflective liquid crystal display device that performs display by reflecting external light is described; however, one embodiment of the present invention is not limited to this type of liquid crystal display device. For example, a light source (e.g., a back light or a side light) may be provided to form a transmissive liquid crystal display device or a transflective liquid crystal display device.

The display device 2501 illustrated in FIG. 49B has the same structure as the display device 2501 illustrated in FIG. 49A except for the following points.

The pixel 2505 in the display device 2501 illustrated in FIG. 49B includes a liquid crystal element 2551 and the transistor 2502 t that can supply electric power to the liquid crystal element 2551.

The liquid crystal element 2551 includes a lower electrode (also referred to as a pixel electrode), an upper electrode, and a liquid crystal layer 2529 between the lower electrode and the upper electrode. By the application of a voltage between the lower electrode and the upper electrode, the alignment state of the liquid crystal layer 2529 in the liquid crystal element 2551 can be changed. Furthermore, in the liquid crystal layer 2529, a spacer 2530 a and a spacer 2530 b are provided. Although not illustrated in FIG. 49B, an alignment film may be provided on each of the upper electrode and the lower electrode on the side in contact with the liquid crystal layer 2529.

As the liquid crystal layer 2529, thermotropic liquid crystal, low-molecular liquid crystal, high-molecular liquid crystal, polymer dispersed liquid crystal, ferroelectric liquid crystal, or anti-ferroelectric liquid crystal can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions. In the case of employing a horizontal electric field mode liquid crystal display device, liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. In the case where a liquid crystal exhibiting a blue phase is used, an alignment film is not necessarily provided, so that rubbing treatment is also unnecessary. Accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device in the manufacturing process can be reduced.

The spacers 2530 a and 2530 b are formed by selectively etching an insulating film. The spacers 2530 a and 2530 b are provided in order to control the distance between the substrate 2510 and the substrate 2570 (the cell gap). Note that the spacers 2530 a and 2530 b may have different sizes from each other and are preferably have a columnar or spherical shape. Although the spacers 2530 a and 2530 b are provided on the substrate 2570 side in the non-limiting structure in FIG. 49B, they may be provided on the substrate 2510 side.

The upper electrode of the liquid crystal element 2551 is provided on the substrate 2570 side. An insulating layer 2531 is provided between the upper electrode and the coloring layer 2567 and the light-blocking layer 2568. The insulating layer 2531 has a function of covering the roughness caused by the coloring layer 2567 and the light-blocking layer 2568 to provide a flat surface. As the insulating layer 2531, an organic resin film may be used, for example. The lower electrode of the liquid crystal element 2551 has a function of a reflective electrode. The display device 2501 illustrated in FIG. 49B is of a reflective type which performs display by reflecting external light at the lower electrode and making the light pass through the coloring layer 2567. Note that in the case of forming a transmissive liquid crystal display device, a transparent electrode is provided as the lower electrode.

The display device 2501 illustrated in FIG. 49B includes an insulating layer 2522. The insulating layer 2522 covers the transistor 2502 t and the like. The insulating layer 2522 has a function of covering the roughness caused by the pixel circuit to provide a flat surface and a function of forming roughness on the lower electrode of the liquid crystal element. In this way, roughness can be formed on the surface of the lower electrode. Therefore, when external light is incident on the lower electrode, the light is reflected diffusely at the surface of the lower electrode, whereby visibility can be improved. Note that in the case of forming a transmissive liquid crystal display device, a structure without such roughness may be employed.

<5-3. Touch Sensor>

Next, the touch sensor 2595 will be described in detail with reference to FIG. 50. FIG. 50 corresponds to a cross-sectional view taken along dashed-dotted line X3-X4 in FIG. 48B.

The touch sensor 2595 includes the electrodes 2591 and the electrodes 2592 provided in a staggered arrangement on the substrate 2590, an insulating layer 2593 covering the electrodes 2591 and the electrodes 2592, and the wiring 2594 that electrically connects the adjacent electrodes 2591 to each other.

The electrodes 2591 and the electrodes 2592 are formed using a light-transmitting conductive material. As a light-transmitting conductive material, a conductive oxide such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added can be used. Note that a film containing graphene may be used as well. The film containing graphene can be formed, for example, by reducing a film containing graphene oxide. As a reducing method, a method with application of heat or the like can be employed.

The electrodes 2591 and the electrodes 2592 may be formed by, for example, depositing a light-transmitting conductive material on the substrate 2590 by a sputtering method and then removing an unnecessary portion by any of various patterning techniques such as photolithography.

Examples of a material for the insulating layer 2593 include a resin such as an acrylic resin or an epoxy resin, a resin having a siloxane bond, and an inorganic insulating material such as silicon oxide, silicon oxynitride, or aluminum oxide.

Openings reaching the electrodes 2591 are formed in the insulating layer 2593, and the wiring 2594 electrically connects the adjacent electrodes 2591. A light-transmitting conductive material can be favorably used as the wiring 2594 because the aperture ratio of the touch panel can be increased. Moreover, a material with higher conductivity than the conductivities of the electrodes 2591 and 2592 can be favorably used for the wiring 2594 because electric resistance can be reduced.

One electrode 2592 extends in one direction, and a plurality of electrodes 2592 are provided in the form of stripes. The wiring 2594 intersects with the electrode 2592.

Adjacent electrodes 2591 are provided with one electrode 2592 provided therebetween. The wiring 2594 electrically connects the adjacent electrodes 2591.

Note that the plurality of electrodes 2591 are not necessarily arranged in the direction orthogonal to one electrode 2592 and may be arranged to intersect with one electrode 2592 at an angle of more than 0 degrees and less than 90 degrees.

The wiring 2598 is electrically connected to any of the electrodes 2591 and 2592. Part of the wiring 2598 functions as a terminal. For the wiring 2598, a metal material such as aluminum, gold, platinum, silver, nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or an alloy material containing any of these metal materials can be used.

Note that an insulating layer that covers the insulating layer 2593 and the wiring 2594 may be provided to protect the touch sensor 2595.

A connection layer 2599 electrically connects the wiring 2598 to the FPC 2509(2).

As the connection layer 2599, any of various anisotropic conductive films (ACF), anisotropic conductive pastes (ACP), or the like can be used.

<5-4. Touch Panel>

Next, the touch panel 2000 will be described in detail with reference to FIG. 51A. FIG. 51A corresponds to a cross-sectional view taken along dashed-dotted line X5-X6 in FIG. 48A.

In the touch panel 2000 illustrated in FIG. 51A, the display device 2501 described with reference to FIG. 48A and the touch sensor 2595 described with reference to FIG. 50 are attached to each other.

The touch panel 2000 illustrated in FIG. 51A includes an adhesive layer 2597 and an anti-reflective layer 2569 in addition to the components described with reference to FIG. 49A.

The adhesive layer 2597 is provided in contact with the wiring 2594. Note that the adhesive layer 2597 attaches the substrate 2590 to the substrate 2570 so that the touch sensor 2595 overlaps with the display device 2501. The adhesive layer 2597 preferably has a light-transmitting property. A heat curable resin or an ultraviolet curable resin can be used for the adhesive layer 2597. For example, an acrylic resin, a urethane-based resin, an epoxy-based resin, or a siloxane-based resin can be used.

The anti-reflective layer 2569 is positioned in a region overlapping with pixels. As the anti-reflective layer 2569, a circularly polarizing plate can be used, for example.

Next, a touch panel having a structure different from that illustrated in FIG. 51A will be described with reference to FIG. 51B.

FIG. 51B is a cross-sectional view of a touch panel 2001. The touch panel 2001 illustrated in FIG. 51B differs from the touch panel 2000 illustrated in FIG. 51A in the position of the touch sensor 2595 relative to the display device 2501. Different parts are described in detail below, and the above description of the touch panel 2000 is referred to for the other similar parts.

The coloring layer 2567 is positioned under the EL element 2550. The EL element 2550 illustrated in FIG. 51B emits light to the side where the transistor 2502 t is provided. Accordingly, part of light emitted from the EL element 2550 passes through the coloring layer 2567 and is emitted to the outside of the light-emitting module 2580 as indicated by an arrow in FIG. 51B.

The touch sensor 2595 is provided on the substrate 2510 side of the display device 2501.

The adhesive layer 2597 is provided between the substrate 2510 and the substrate 2590 and attaches the touch sensor 2595 to the display device 2501.

As illustrated in FIG. 51A or FIG. 51B, light may be emitted from the light-emitting element through one or both of the substrate 2510 and the substrate 2570.

<5-5. Driving Method of Touch Panel>

Next, an example of a method for driving a touch panel will be described with reference to FIGS. 52A and 52B.

FIG. 52A is a block diagram illustrating the structure of a mutual capacitive touch sensor. FIG. 52A illustrates a pulse voltage output circuit 2601 and a current sensing circuit 2602. Note that in FIG. 52A, six wirings X1 to X6 represent the electrodes 2621 to which a pulse voltage is applied, and six wirings Y1 to Y6 represent the electrodes 2622 that detect changes in current. FIG. 52A also illustrates capacitors 2603 that are each formed in a region where the electrodes 2621 and 2622 overlap with each other. Note that functional replacement between the electrodes 2621 and 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentially applying a pulse voltage to the wirings X1 to X6. By application of a pulse voltage to the wirings X1 to X6, an electric field is generated between the electrodes 2621 and 2622 of the capacitor 2603. When the electric field between the electrodes is shielded, for example, a change occurs in the capacitor 2603 (mutual capacitance). The approach or contact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 2602 is a circuit for detecting changes in current flowing through the wirings Y1 to Y6 that are caused by the change in mutual capacitance in the capacitor 2603. No change in current value is detected in the wirings Y1 to Y6 when there is no approach or contact of a sensing target, whereas a decrease in current value is detected when mutual capacitance is decreased owing to the approach or contact of a sensing target. Note that an integrator circuit or the like is used for sensing of current values.

FIG. 52B is a timing chart showing input and output waveforms in the mutual capacitive touch sensor illustrated in FIG. 52A. In FIG. 52B, sensing of a sensing target is performed in all the rows and columns in one frame period. FIG. 52B shows a period when a sensing target is not sensed (not touched) and a period when a sensing target is sensed (touched). Sensed current values of the wirings Y1 to Y6 are shown as the waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and the waveforms of the wirings Y1 to Y6 change in accordance with the pulse voltage. When there is no approach or contact of a sensing target, the waveforms of the wirings Y1 to Y6 change in accordance with changes in the voltages of the wirings X1 to X6. The current value is decreased at the point of approach or contact of a sensing target and accordingly the waveform of the voltage value changes.

By detecting a change in mutual capacitance in this manner, the approach or contact of a sensing target can be sensed.

<5-6. Sensor Circuit>

Although FIG. 52A illustrates a passive type touch sensor in which only the capacitor 2603 is provided at the intersection of wirings as a touch sensor, an active type touch sensor including a transistor and a capacitor may be used. FIG. 53 illustrates an example of a sensor circuit included in an active type touch sensor.

The sensor circuit in FIG. 53 includes the capacitor 2603 and transistors 2611, 2612, and 2613.

A signal G2 is input to a gate of the transistor 2613. A voltage VRES is applied to one of a source and a drain of the transistor 2613, and one electrode of the capacitor 2603 and a gate of the transistor 2611 are electrically connected to the other of the source and the drain of the transistor 2613. One of a source and a drain of the transistor 2611 is electrically connected to one of a source and a drain of the transistor 2612, and a voltage VSS is applied to the other of the source and the drain of the transistor 2611. A signal G1 is input to a gate of the transistor 2612, and a wiring ML is electrically connected to the other of the source and the drain of the transistor 2612. The voltage VSS is applied to the other electrode of the capacitor 2603.

Next, the operation of the sensor circuit in FIG. 53 will be described. First, a potential for turning on the transistor 2613 is supplied as the signal G2, and a potential with respect to the voltage VRES is thus applied to the node n connected to the gate of the transistor 2611. Then, a potential for turning off the transistor 2613 is applied as the signal G2, whereby the potential of the node n is maintained.

Then, mutual capacitance of the capacitor 2603 changes owing to the approach or contact of a sensing target such as a finger, and accordingly the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor 2612 is supplied as the signal G1. A current flowing through the transistor 2611, that is, a current flowing through the wiring ML is changed in accordance with the potential of the node n. By sensing this current, the approach or contact of a sensing target can be sensed.

In each of the transistors 2611, 2612, and 2613, any of the transistors described in the above embodiments can be used. In particular, it is preferable to use any of the transistors described in the above embodiments as the transistor 2613 because the potential of the node n can be held for a long time and the frequency of operation of resupplying VRES to the node n (refresh operation) can be reduced.

The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments.

Embodiment 6

In this embodiment, a display device including a semiconductor device of one embodiment of the present invention and a method for driving the display device are described with reference to FIGS. 54A and 54B, FIGS. 55A and 55B, FIGS. 56A to 56E, and FIGS. 57A to 57E.

Note that the display device of one embodiment of the present invention may include an information processing unit, an arithmetic unit, a memory unit, a display unit, an input unit, and the like.

In the case where the display device of one embodiment of the present invention continuously displays the same image (still image), power consumption can be reduced by reducing the frequency of writing signals (also referred to as “refresh”) for the same image. Note that the rate at which the refresh is performed is referred to as refresh rate (also referred to as scan frequency or vertical synchronization frequency). The display device that reduces eye strain by reducing the refresh rate is described below.

The eye strain is divided into two categories: nerve strain and muscle strain. The nerve strain is caused by prolonged looking at light emitted from a display device or blinking images. This is because the brightness stimulates and fatigues a retina, optic nerves, and a brain. The muscle strain is caused by overuse of a ciliary muscle which works for adjusting the focus.

FIG. 54A is a schematic diagram showing display on a conventional display device. As illustrated in FIG. 54A, for the display of the conventional display device, image rewriting is performed 60 times every second. A prolonged looking at such a screen might stimulate a retina, optic nerves, and a brain of a user and lead to eye strain.

In a display device of one embodiment of the present invention, a transistor using an oxide semiconductor, for example, a transistor using CAAC-OS is used in a pixel portion of the display device. The off-state current of the transistor is extremely low. Therefore, the luminance of the display device can be kept even when the refresh rate of the display device is lowered.

Thus, for example, the number of times of image writing can be reduced to once every five seconds as illustrated in FIG. 54B. The same image can be displayed for as long as possible and flickers on a screen perceived by a user can be reduced. Consequently, a stimulus to the retina or the nerve of an eye or the brain of the user is relieved, resulting in less nervous fatigue.

In the case where the size of one pixel is large (e.g., the resolution is less than 150 ppi), a blurred character is displayed by a display device as shown in FIG. 55A. When users look at the blurred character displayed on the display device for a long time, their ciliary muscles keep working to adjust the focus in a state where adjusting the focus is difficult, which might lead to eye strain.

In contrast, the display device of one embodiment of the present invention has a small-size pixel and thus can display high-resolution images, so that a precise and smooth image can be displayed as shown in FIG. 55B. In this case, the ciliary muscle can easily focus the eye on the character, so that the user's muscular fatigue is reduced. When the resolution of the display device is 150 ppi or more, preferably 200 ppi or more, more preferably 300 ppi or more, the user's muscular fatigue can be effectively reduced.

Methods for quantifying eye fatigue have been studied. For example, critical flicker (fusion) frequency (CFF) is known as an indicator for evaluating nervous fatigue. Further, focus adjustment time, near point distance, and the like are known as indicators for evaluating muscular fatigue.

Others methods for evaluating eye fatigue include electroencephalography, thermography, counting the number of times of blinking, measuring the amount of tears, measuring the speed of contractile response of the pupil, and questionnaires for surveying subjective symptoms.

The method for driving the display device of one embodiment of the present invention can be evaluated by any of the variety of methods above, for example.

6. Method for Driving Display Device

Now, a method for driving the display device of one embodiment of the present invention is described with reference to FIGS. 56A to 56E.

Display Example of Image Information

An example of displaying two images including different image data by being transferred is described below.

FIG. 56A illustrates an example in which a window 451 and a first image 452 a which is a still image displayed in the window 451 are displayed on a display portion 450.

At this time, display is preferably performed at a first refresh rate. Note that the first refresh rate can be higher than or equal to 1.16×10⁻⁵ Hz (about once per day) and lower than or equal to 1 Hz, higher than or equal to 2.78×10⁻⁴ Hz (about once per hour) and lower than or equal to 0.5 Hz, or higher than or equal to 1.67×10⁻² Hz (about once per hour) and lower than or equal to 0.1 Hz.

When frequency of rewriting an image is reduced by setting the first refresh rate to an extremely low value, display substantially without flicker can be achieved, and eye fatigue of a user can be effectively reduced.

The window 451 is displayed by, for example, executing application software for image display and includes a display region where an image is displayed.

Further, in a lower part of the window 451, a button 453 for switching a displayed image data to a different image data is provided. When a user performs operation in which the button 453 is selected, an instruction of transferring an image can be supplied to the information processing unit of the display device.

Note that the operation method performed by the user may be set in accordance with the input unit. For example, in the case where a touch panel provided to overlap with the display portion 450 is used as the input unit, input operation can be performed by touching the button 453 with a finger or a stylus or performing gesture operation where an image is made to slide. In the case where the input operation is performed with gesture or sound, the button 453 is not necessarily displayed.

When the information processing unit of the display device receives the instruction of transferring an image, transfer of the image displayed in the window 451 starts (see FIG. 56B).

Note that in the case where display is performed at the first refresh rate in the state of FIG. 56A, the refresh rate is preferably changed to a second refresh rate before transfer of the image starts. The second refresh rate is a value necessary for displaying a moving image. For example, the second refresh rate can be higher than or equal to 30 Hz and lower than or equal to 960 Hz, preferably higher than or equal to 60 Hz and lower than or equal to 960 Hz, further preferably higher than or equal to 75 Hz and lower than or equal to 960 Hz, still further preferably higher than or equal to 120 Hz and lower than or equal to 960 Hz, still further preferably higher than or equal to 240 Hz and lower than or equal to 960 Hz.

When the second refresh rate is set to a higher value than the first refresh rate, a moving image can be displayed further smoothly and naturally. In addition, flicker which accompanies rewriting of data is less likely to be recognized by a user, whereby eye fatigue of a user can be reduced.

At this time, an image where the first image 452 a and a second image 452 b that is to be displayed next are combined is displayed in the window 451. The combined image is transferred unidirectionally (leftward in this case), and part of a region is displayed in the window 451.

Further, when the combined image transfers, luminance of the image displayed in the window 451 is gradually lowered from the initial luminance at the time of the state in FIG. 56A.

FIG. 56C illustrates a state where the image displayed in the window 451 reaches a position of the predetermined coordinates. Thus, the luminance of the image displayed in the window 451 at this time is lowest.

Note that the predetermined coordinates in FIG. 56C is set so that half of the first image 452 a and half of the second image 452 b are displayed; however, the coordinates are not limited to the above, and it is preferable that the coordinates be set freely by a user.

For example, the predetermined coordinates may be set so that the ratio of the distance between the initial coordinates and the predetermined coordinates to the distance between the initial coordinates and the final coordinates is higher than 0 and lower than 1.

In addition, it is also preferable that luminance when the image reaches the position of the predetermined coordinates be set freely by a user. For example, the ratio of the luminance when the image reaches the position of the predetermined coordinates to the initial luminance may be higher than 0 and lower than 1, preferably higher than or equal to 0 and lower than or equal to 0.8, further preferably higher than or equal to 0 and lower than or equal to 0.5.

Next, in the window 451, the combined image transfers with the luminance increasing gradually (FIG. 56D)

FIG. 56E illustrates a state when the combined image reaches the position of the final coordinates. In the window 451, only the second image 452 b is displayed with luminance equal to the initial luminance.

Note that after the transfer of the image is completed, the refresh rate is preferably changed from the second refresh rate to the first refresh rate.

Since the luminance of the image is lowered in such a display mode, even when a user follows the motion of the image with his/her eyes, the user is less likely to suffer from eye fatigue. Thus, by such a driving method, eye-friendly display can be achieved.

Display Example of Document Information

Next, an example in which document information whose dimension is larger than a display window is displayed by scrolling is described below.

FIG. 57A illustrates an example in which a window 455 and part of document information 456 which is a still image displayed in the window 455 are displayed on the display portion 450.

At this time, display is preferably performed at the first refresh rate.

The window 455 is displayed by, for example, executing application software for document display, application software for document preparation, or the like and includes a display region where document information is displayed.

The dimension of an image of the document information 456 is larger than the display region of the window 455 in the longitudinally direction. That is, part of the document information 456 is displayed in the window 455. Further, as illustrated in FIG. 57A, the window 455 may be provided with a scroll bar 457 which indicates which part in the whole of the document information 456 is displayed.

When an instruction of transferring an image (here, also referred to as scroll instruction) is supplied to the display device by the input unit, transfer of the document information 456 starts (FIG. 57B). In addition, luminance of the displayed image is gradually lowered.

Note that in the case where display is performed at the first refresh rate in the state of FIG. 57A, the refresh rate is preferably changed to the second refresh rate before transfer of the document information 456.

In this state, not only the luminance of the image displayed in the window 455 but the luminance of the whole image displayed on the display portion 450 is lowered.

FIG. 57C illustrates a state when the document information 456 reaches a position of the predetermined coordinates. At this time, the luminance of the whole image displayed on the display portion 450 is the lowest.

Then, the document information 456 is displayed in the window 455 while being transferred (FIG. 57D). Under this condition, the luminance of the whole image displayed on the display portion 450 is gradually increased.

FIG. 57E illustrates a state where the document information 456 reaches a position of the final coordinates. In the window 455, a region of the document information 456, which is different from the region displayed in an initial state, is displayed with luminance equal to the initial luminance.

Note that after transfer of the document information 456 is completed, the refresh rate is preferably changed to the first refresh rate.

Since the luminance of the image is lowered in such a display mode, even when a user follows the motion of the image with his/her eyes, the user is less likely to suffer from eye fatigue. Thus, by such a driving method, eye-friendly display can be achieved.

In particular, display of document information or the like, which has relatively high contrast ratio, gives a user eye fatigue significantly; thus, it is preferable to apply such a driving method to the display of document information.

This embodiment can be combined with any of the other embodiments disclosed in this specification as appropriate.

Embodiment 7

In this embodiment, a display module, electronic devices, and a display device which include a semiconductor device of one embodiment of the present invention will be described with reference to FIG. 58, FIGS. 59A to 59G, and FIGS. 60A and 60B.

<7-1. Display Module>

In a display module 8000 illustrated in FIG. 58, a touch panel 8004 connected to an FPC 8003, a display panel 8006 connected to an FPC 8005, a backlight 8007, a frame 8009, a printed board 8010, and a battery 8011 are provided between an upper cover 8001 and a lower cover 8002.

The oxide semiconductor film or the semiconductor device of one embodiment of the present invention can be used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002 can be changed as appropriate in accordance with the sizes of the touch panel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitive touch panel and can be formed to overlap with the display panel 8006. A counter substrate (sealing substrate) of the display panel 8006 can have a touch panel function. A photosensor may be provided in each pixel of the display panel 8006 to form an optical touch panel.

The backlight 8007 includes light sources 8008. Note that although a structure in which the light sources 8008 are provided over the backlight 8007 is illustrated in FIG. 58, one embodiment of the present invention is not limited to this structure. For example, a structure in which the light sources 8008 are provided at an end portion of the backlight 8007 and a light diffusion plate is further provided may be employed. Note that the backlight 8007 need not be provided in the case where a self-luminous light-emitting element such as an organic EL element is used or in the case where a reflective panel or the like is employed.

The frame 8009 protects the display panel 8006 and also functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board 8010. The frame 8009 may function as a radiator plate.

The printed board 8010 is provided with a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or a power source using the battery 8011 provided separately may be used. The battery 8011 can be omitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.

<7-2. Electronic Device>

FIGS. 59A to 59G illustrate electronic devices. These electronic devices can each include a housing 9000, a display portion 9001, a speaker 9003, an operation key 9005 (including a power switch or an operation switch), a connection terminal 9006, a sensor 9007 (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), a microphone 9008, and the like.

The electronic devices illustrated in FIGS. 59A to 59G can have a variety of functions, for example, a function of displaying a variety of information (a still image, a moving image, a text image, and the like) on the display portion, a touch panel function, a function of displaying a calendar, the date, the time, and the like, a function of controlling processing with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, a function of reading a program or data stored in a storage medium and displaying the program or data on the display portion, and the like. Note that functions of the electronic devices illustrated in FIGS. 59A to 59G are not limited thereto, and the electronic devices can have a variety of functions. Although not illustrated in FIGS. 59A to 59G, the electronic devices may each have a plurality of display portions. The electronic devices may each have a camera or the like and a function of taking a still image, a function of taking a moving image, a function of storing the taken image in a storage medium (an external storage medium or a storage medium incorporated in the camera), a function of displaying the taken image on the display portion, and the like.

The electronic devices illustrated in FIGS. 59A to 59G will be described in detail below.

FIG. 59A is a perspective view of a portable information terminal 9100. The display portion 9001 of the portable information terminal 9100 is flexible and thus can be incorporated along the curved surface of the housing 9000. Furthermore, the display portion 9001 includes a touch sensor, and operation can be performed by touching a screen with a finger, a stylus, or the like. For example, by touching an icon displayed on the display portion 9001, an application can be started.

FIG. 59B is a perspective view of a portable information terminal 9101. The portable information terminal 9101 functions as, for example, one or more of a telephone set, a notebook, an information browsing system, and the like. Specifically, the portable information terminal 9101 can be used as a smartphone. Note that the speaker 9003, the connection terminal 9006, the sensor 9007, and the like, which are not illustrated in FIG. 59B, can be positioned in the portable information terminal 9101 as in the portable information terminal 9100 illustrated in FIG. 59A. The portable information terminal 9101 can display characters and image information on its plurality of surfaces. For example, three operation buttons 9050 (also referred to as operation icons, or simply, icons) can be displayed on one surface of the display portion 9001. Furthermore, information 9051 indicated by dashed rectangles can be displayed on another surface of the display portion 9001. Examples of the information 9051 include display indicating reception of an incoming e-mail, social networking service (SNS) message, call, or the like; the title and sender of an e-mail, SNS message, or the like; the date; the time; remaining battery; the strength of an antenna; and the like. Instead of the information 9051, the operation buttons 9050 or the like may be displayed in the position where the information 9051 is displayed.

FIG. 59C is a perspective view of a portable information terminal 9102. The portable information terminal 9102 has a function of displaying information on three or more surfaces of the display portion 9001. Here, information 9052, information 9053, and information 9054 are displayed on different surfaces. For example, a user of the portable information terminal 9102 can see the display (here, the information 9053) with the portable information terminal 9102 put in a breast pocket of his/her clothes. Specifically, a caller's phone number, name, or the like of an incoming call is displayed in the position that can be seen from above the portable information terminal 9102. Thus, the user can see the display without taking out the portable information terminal 9102 from the pocket and decide whether to answer the call.

FIG. 59D is a perspective view of a watch-type portable information terminal 9200. The portable information terminal 9200 is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, Internet communication, and computer games. The display surface of the display portion 9001 is curved, and display can be performed on the curved display surface. The portable information terminal 9200 can employ near field communication conformable to a communication standard. For example, hands-free calling can be achieved with mutual communication between the portable information terminal 9200 and a headset capable of wireless communication. Moreover, the portable information terminal 9200 includes the connection terminal 9006, and data can be directly transmitted to and received from another information terminal via a connector. Charging through the connection terminal 9006 is also possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal 9006.

FIGS. 59E, 59F, and 59G are perspective views of a foldable portable information terminal 9201. FIG. 57E is a perspective view of the foldable portable information terminal 9201 that is opened. FIG. 59F is a perspective view of the foldable portable information terminal 9201 that is being opened or being folded. FIG. 59G is a perspective view of the foldable portable information terminal 9201 that is folded. The portable information terminal 9201 is highly portable when folded. When the portable information terminal 9201 is opened, a seamless large display region provides high browsability. The display portion 9001 of the portable information terminal 9201 is supported by three housings 9000 joined together by hinges 9055. By folding the portable information terminal 9201 at a connection portion between two housings 9000 with the hinges 9055, the portable information terminal 9201 can be reversibly changed in shape from the opened state to the folded state. For example, the portable information terminal 9201 can be bent with a radius of curvature of greater than or equal to 1 mm and less than or equal to 150 mm.

FIGS. 60A and 60B are perspective views of a display device including a plurality of display panels. Note that the plurality of display panels are wound in the perspective view in FIG. 60A, and are unwound in the perspective view in FIG. 60B.

A display device 9500 illustrated in FIGS. 60A and 60B includes a plurality of display panels 9501, a hinge 9511, and a bearing 9512. The plurality of display panels 9501 each include a display region 9502 and a light-transmitting region 9503.

Each of the plurality of display panels 9501 is flexible. Two adjacent display panels 9501 are provided so as to partly overlap with each other. For example, the light-transmitting regions 9503 of the two adjacent display panels 9501 can be overlapped each other. A display device having a large screen can be obtained with the plurality of display panels 9501. The display device is highly versatile because the display panels 9501 can be wound depending on its use.

Moreover, although the display regions 9502 of the adjacent display panels 9501 are separated from each other in FIGS. 60A and 60B, without limitation to this structure, the display regions 9502 of the adjacent display panels 9501 may overlap with each other without any space so that a continuous display region 9502 is obtained, for example.

The electronic devices described in this embodiment each include the display portion for displaying some sort of data. Note that the semiconductor device of one embodiment of the present invention can also be used for an electronic device that does not have a display portion. The structure in which the display portion of the electronic device described in this embodiment is flexible and display can be performed on the curved display surface or the structure in which the display portion of the electronic device is foldable is described as an example; however, the structure is not limited thereto, and a structure in which the display portion of the electronic device is not flexible and display is performed on a plane portion may be employed.

The structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

Embodiment 8

In this embodiment, a deposition apparatus which can be used for manufacturing the display module of one embodiment of the present invention will be described with reference to FIG. 61.

FIG. 61 illustrates a deposition apparatus 3000 which can be used for manufacturing the display module of one embodiment of the present invention. Note that the deposition apparatus 3000 is an example of a batch-type ALD apparatus.

8-1. Structural Example of Deposition Apparatus

The deposition apparatus 3000 described in this embodiment includes a deposition chamber 3180 and a control portion 3182 connected to the deposition chamber 3180 (see FIG. 61).

The control portion 3182 includes a control unit (not illustrated) which supplies control signals and flow rate controllers 3182 a, 3182 b, and 3182 c to which the control signals are supplied. For example, high-speed valves can be used as the flow rate controllers. Specifically, flow rates can be precisely controlled by using ALD valves or the like. The control portion 3182 also includes a heating mechanism 3182 h which controls the temperatures of the flow rate controllers and pipes.

The flow rate controller 3182 a is supplied with a control signal, a first source material, and an inert gas and has a function of supplying the first source material or the inert gas in accordance with the control signal.

The flow rate controller 3182 b is supplied with a control signal, a second source material, and an inert gas and has a function of supplying the second source material or the inert gas in accordance with the control signal.

The flow rate controller 3182 c is supplied with a control signal and has a function of connecting to an evacuation unit 3185 in accordance with the control signal.

[Source Material Supply Portion]

A source material supply portion 3181 a has a function of supplying the first source material and is connected to the first flow rate controller 3182 a.

A source material supply portion 3181 b has a function of supplying the second source material and is connected to the second flow rate controller 3182 b.

A vaporizer, a heating unit, or the like can be used as each of the source material supply portions. Thus, a gaseous source material can be generated from a solid or liquid source material.

Note that the number of source material supply portions is not limited to two and may be three or more.

[Source Material]

Any of a variety of substances can be used as the first source material. For example, a volatile organometallic compound, a metal alkoxide, or the like can be used as the first source material. Any of a variety of substances which react with the first source material can be used as the second source material. For example, a substance which contributes to an oxidation reaction, a substance which contributes to a reduction reaction, a substance which contributes to an addition reaction, a substance which contributes to a decomposition reaction, a substance which contributes to a hydrolysis reaction, or the like can be used as the second source material.

Furthermore, a radical or the like can be used. For example, plasma obtained by supplying a source material to a plasma source or the like can be used. Specifically, an oxygen radical, a nitrogen radical, or the like can be used.

The second source material combined with the first source material is preferably a source material which reacts at a temperature close to room temperature. For example, a source material which reacts at a temperature higher than or equal to room temperature and lower than or equal to 200° C., preferably higher than or equal to 50° C. and lower than or equal to 150° C., is preferable.

[Evacuation Unit]

The evacuation unit 3185 has an evacuating function and is connected to the third flow rate controller 3182 c. Note that a trap for capturing the source material to be evacuated may be provided between an outlet port 3184 and the third flow rate controller 3182 c. The evacuated gas is removed by using a removal unit.

[Control Portion]

The control portion 3182 supplies the control signal which controls the flow rate controllers, a control signal which controls the heating mechanism, or the like. For example, in a first step, the first source material is supplied to a surface of a process member. Then, in a second step, the second source material which reacts with the first source material is supplied. Accordingly, a reaction product of the first source material and the second source material can be deposited onto a surface of a process member 3010.

Note that the amount of the reaction product to be deposited onto the surface of the process member 3010 can be controlled by repetition of the first step and the second step.

Note that the amount of the first source material to be supplied to the process member 3010 is limited by the maximum possible amount of adsorption on the surface of the process member 3010. For example, conditions are selected so that a monomolecular layer of the first source material is formed on the surface of the process member 3010, and the formed monomolecular layer of the first source material is reacted with the second source material, whereby a significantly uniform layer containing the reaction product of the first source material and the second source material can be formed.

Accordingly, a variety of materials can be deposited on a surface of the process member 3010 even when the surface has a complicated structure. For example, a film having a thickness greater than or equal to 3 nm and less than or equal to 200 nm can be formed on the process member 3010.

In the case where, for example, a small hole called a pinhole or the like is formed in the surface of the process member 3010, the pinhole can be filled by depositing a material into the pinhole.

The remainder of the first source material or the second source material is evacuated from the deposition chamber 3180 with use of the evacuation unit 3185. For example, the evacuation may be performed while an inert gas such as argon or nitrogen is introduced.

[Deposition Chamber]

The deposition chamber 3180 includes an inlet port 3183 from which the first source material, the second source material, and the inert gas are supplied and the outlet port 3184 from which the first source material, the second source material, and the inert gas are evacuated.

The deposition chamber 3180 includes a support portion 3186 which has a function of supporting one or a plurality of process members 3010, a heating mechanism 3187 which has a function of heating the one or plurality of process members 3010, and a door 3188 which has a function of opening or closing to load and unload the one or plurality of process members 3010.

For example, a resistive heater, an infrared lamp, or the like can be used as the heating mechanism 3187. The heating mechanism 3187 has a function of heating up, for example, to 80° C. or higher, 100° C. or higher, or 150° C. or higher. The heating mechanism 3187 heats the one or plurality of process members 3010 to a temperature higher than or equal to room temperature and lower than or equal to 200° C., preferably higher than or equal to 50° C. and lower than or equal to 150° C.

The deposition chamber 3180 may also include a pressure regulator and a pressure detector.

[Support Portion]

The support portion 3186 supports the one or plurality of process members 3010. Accordingly, an insulating film, for example, can be formed over the one or plurality of process members 3010 in each treatment.

8-2. Example of Film

An example of a film which can be formed with the deposition apparatus 3000 described in this embodiment will be described.

For example, a film including an oxide, a nitride, a fluoride, a sulfide, a ternary compound, a metal, or a polymer can be formed.

For example, the film can be formed with a material including aluminum oxide, hafnium oxide, aluminum silicate, hafnium silicate, lanthanum oxide, silicon oxide, strontium titanate, tantalum oxide, titanium oxide, zinc oxide, niobium oxide, zirconium oxide, tin oxide, yttrium oxide, cerium oxide, scandium oxide, erbium oxide, vanadium oxide, indium oxide, or the like.

For example, the film can be formed with a material including aluminum nitride, hafnium nitride, silicon nitride, tantalum nitride, titanium nitride, niobium nitride, molybdenum nitride, zirconium nitride, gallium nitride, or the like.

For example, the film can be formed with a material including copper, platinum, ruthenium, tungsten, iridium, palladium, iron, cobalt, nickel, or the like.

For example, the film can be formed with a material including zinc sulfide, strontium sulfide, calcium sulfide, lead sulfide, calcium fluoride, strontium fluoride, zinc fluoride, or the like.

For example, the film can be formed with a material which includes a nitride containing titanium and aluminum, an oxide containing titanium and aluminum, an oxide containing aluminum and zinc, a sulfide containing manganese and zinc, a sulfide containing cerium and strontium, an oxide containing erbium and aluminum, an oxide containing yttrium and zirconium, or the like.

This embodiment can be combined with any of the other embodiments in this specification as appropriate.

Example 1

In this example, a transistor corresponding to the transistor 150 shown in FIGS. 2A to 2C was manufactured and the drain current-gate voltage characteristics (I_(d)-V_(g) characteristics) of the transistor were evaluated. Sample C1 described below was manufactured for the evaluation in this example. Note that Sample C1 includes a transistor of one embodiment of the present invention. Sample C1 includes a transistor having a channel length L of 2 μm and a channel width W of 50 μm, a transistor having a channel length L of 3 μm and a channel width W of 50 μm, and a transistor having a channel length L of 6 μm and a channel width W of 50 μm. The transistors were fabricated 10 for each kind.

The samples formed in this example are described below. Note that the reference numerals used for the transistor 150 in FIGS. 2A to 2C are used in the following description.

<1-1. Formation Method of Sample C1>

First, the conductive film 104 was formed over the substrate 102. As the substrate 102, a glass substrate was used. As the conductive film 104, a 100-nm-thick tungsten film was formed with a sputtering apparatus.

Next, the insulating films 106 and 107 were formed over the substrate 102 and the conductive film 104. As the insulating film 106, a 400-nm-thick silicon nitride film was formed with a PECVD apparatus. As the insulating film 107, a 50-nm-thick silicon oxynitride film was formed with a PECVD apparatus.

The insulating film 106 was deposited as follows. First, a 50-nm-thick silicon nitride film was deposited under the conditions where the substrate temperature was 350° C.; a silane gas at a flow rate of 200 sccm, a nitrogen gas at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccm were introduced into a chamber; the pressure was set to 100 Pa; and an RF power of 2000 W was supplied between parallel-plate electrodes placed in a PECVD apparatus. Then, the flow rate of an ammonia gas was changed to 2000 sccm to deposit a 300-nm-thick silicon nitride film. Finally, the flow rate of an ammonia gas was changed to 100 sccm to deposit a 50-nm-thick silicon nitride film.

The insulating film 107 was deposited under the conditions where the substrate temperature was 350° C., a silane gas at a flow rate of 20 sccm and a dinitrogen monoxide gas at a flow rate of 3000 sccm were introduced into a chamber, the pressure was 40 Pa, and an RF power of 100 W was supplied between parallel-plate electrodes provided in a PECVD apparatus.

Then, the oxide semiconductor film 108 was formed over the insulating film 107. As the oxide semiconductor film 108, the oxide semiconductor film 108 b and the oxide semiconductor film 108 c were successively formed in a vacuum with a sputtering apparatus.

As the oxide semiconductor film 108 b, a 10-nm-thick IGZO film was formed under the conditions where the substrate temperature was 170° C., an argon gas at a flow rate of 140 sccm and an oxygen gas at a flow rate of 60 sccm were introduced into a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W was applied to a polycrystalline metal oxide sputtering target (having an atomic ratio of In:Ga:Zn=4:2:4.1).

As the oxide semiconductor film 108 c, a 15-nm-thick IGZO film was formed under the conditions where the substrate temperature was 170° C., an argon gas at a flow rate of 100 sccm and an oxygen gas at a flow rate of 100 sccm were introduced into a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W was applied to a polycrystalline metal oxide sputtering target (having an atomic ratio of In:Ga:Zn=1:1:1.2).

Next, the conductive films 112 a and 112 b were formed over the insulating film 107 and the oxide semiconductor film 108. As the conductive films 112 a and 112 b, a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a 100-nm-thick titanium film were successively formed in vacuum with a sputtering apparatus.

Next, a surface of the oxide semiconductor film 108 (on the back-channel side) was cleaned. As the cleaning method, a phosphoric acid solution obtained by diluting phosphoric acid (concentration of 85 vol %) 100 times with water was applied to the oxide semiconductor film 108 and the conductive films 112 a and 112 b with a spin cleaning apparatus for 15 seconds.

After that, the insulating film 114 and the insulating film 116 were formed over the insulating film 107, the oxide semiconductor film 108, and the conductive films 112 a and 112 b. As the insulating film 114, a 50-nm-thick silicon oxynitride film was formed with a PECVD apparatus. As the insulating film 116, a 400-nm-thick silicon oxynitride film was formed with a PECVD apparatus. Note that the insulating film 114 and the insulating film 116 were formed successively in vacuum with a PECVD apparatus.

The insulating film 114 was deposited under the conditions where the substrate temperature was 220° C., a silane gas at a flow rate of 50 sccm and a dinitrogen monoxide gas at a flow rate of 2000 sccm were introduced into a chamber, the pressure was 20 Pa, and an RF power of 100 W was supplied between parallel-plate electrodes provided in a PECVD apparatus. The insulating film 116 was deposited under the conditions where the substrate temperature was 220° C., a silane gas at a flow rate of 160 sccm and a dinitrogen monoxide gas at a flow rate of 4000 sccm were introduced into a chamber, the pressure was 200 Pa, and an RF power of 1500 W was supplied between parallel-plate electrodes provided in a PECVD apparatus.

Then, first heat treatment was performed at 350° C. in a nitrogen gas atmosphere for 1 hour.

Next, the opening portion 152 a reaching the conductive film 112 b and the opening portions 152 b and 152 c reaching the conductive film 104 were formed. The opening portions 152 a, 152 b, and 152 c were formed with a dry etching apparatus.

Next, an oxide semiconductor film was formed over the insulating film 116 to cover the opening portions 152 a, 152 b, and 152 c, and processed to form the oxide semiconductor films 120 a and 120 b. As the oxide semiconductor films 120 a and 120 b, a 100-nm-thick IGZO film was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W was applied to a polycrystalline metal oxide sputtering target (having an atomic ratio of In:Ga:Zn=4:2:4.1).

Then, the insulating film 118 was formed over the insulating film 116 and the oxide semiconductor films 120 a and 120 b. As the insulating film 118, a 100-nm-thick silicon nitride film was formed with a PECVD apparatus. The insulating film 118 was deposited under the conditions where the substrate temperature was 350° C., a silane gas at a flow rate of 50 sccm, a nitrogen gas at a flow rate of 5000 sccm, and an ammonia gas at a flow rate of 100 sccm were introduced into a chamber, the pressure was 100 Pa, and high-frequency power of 1000 W was supplied between parallel-plate electrodes provided in a PECVD apparatus with the use of a 27.12 MHz high-frequency power source.

Then, second heat treatment was performed at 250° C. under a nitrogen atmosphere for 1 hour.

Through the above process, Sample C1 of this example was manufactured. Note that the highest temperature through the fabrication of Sample C1 was 350° C.

<1-2. I_(d)-V_(g) Characteristics>

Next, the I_(d)-V_(g) characteristics of Sample C1 were measured. In measuring the I_(d)-V_(g) characteristics, a voltage (V_(g), V_(bg)) applied to the conductive film 104 and the oxide semiconductor film 120 b that respectively function as the first gate electrode and the second gate electrode of the transistor 150 was changed from −15 V to +20 V in increments of 0.25 V. A voltage (V_(s)) applied to the conductive film 112 a functioning as a source electrode was 0 V (comm), and a voltage (V_(d)) applied to the conductive film 112 b functioning as a drain electrode was 0.1 V and 20 V.

FIGS. 62A and 62B and FIG. 63 show the measurement results of the I_(d)-V_(g) characteristics of Sample C1. In FIGS. 62A and 62B and FIG. 63, the first vertical axis indicates I_(d) (A), the second vertical axis indicates μFE (cm²/V_(s)), and the horizontal axis indicates V_(g) (V).

FIG. 62A shows the measurement results of transistors each having a channel length L of 2 μm and a channel width W of 50 μm. FIG. 62B shows the measurement results of transistors each having a channel length L of 3 μm and a channel width W of 50 μm. FIG. 63 shows the measurement results of transistors each having a channel length L of 6 μm and a channel width W of 50 μm.

The results of FIGS. 62A and 62B and FIG. 63 show that Sample C1 includes a transistor which exhibits high field-effect mobility and small variation in electrical characteristics.

<1-3. Bias-Temperature Stress Test (GBT Test)>

Next, the reliability of Sample C1 was evaluated. As the reliability evaluation, a GBT test was used.

The GBT tests in this example were performed under the conditions where the gate voltage (V_(g)) was ±30 V; the drain voltage (V_(d)) and the source voltage (V_(s)) were 0 V (COMMON); the stress temperature was 60° C.; the time for stress application was 1 hour; and two kinds of measurement environments, a dark environment and a photo environment (irradiation with light having approximately 10000 lx with a white LED), were employed. In other words, the source electrode and the drain electrode of the transistor were set at the same potential, and a potential different from that of the source and drain electrodes was applied to the gate electrode for a certain time (1 hour, here).

A case where the potential applied to the gate electrode is higher than that of the source and drain electrodes is called positive stress, and a case where the potential applied to the gate electrode is lower than that of the source and drain electrodes is called negative stress. Thus, the reliability evaluation was performed under four conditions in total, i.e., positive GBT (Dark), negative GBT (Dark), positive GBT (Light irradiation), and negative GBT (Light irradiation). Note that positive GBT (Dark) can be referred to as PBTS (Positive Bias Temperature Stress), negative GBT (Dark) as NBTS (Negative Bias Temperature Stress), positive GBT (Light irradiation) as PBITS (Positive Bias Illuminations Temperature Stress), negative GBT (Light irradiation) as NBITS (Negative Bias Illuminations Temperature Stress).

FIG. 64A shows GBT test results of Sample C1. In FIG. 64A, the vertical axis indicates the amount of change in threshold voltage (ΔV_(th)) of the transistor and the amount of change in shift value (ΔShift), and the horizontal axis indicates the name of conditions. Note that the shift value means, in the drain current (I_(d))-gate voltage (V_(g)) characteristics of the transistor, the gate voltage (V_(g)) at a point of intersection of an axis of 1×10⁻¹² A and a tangent line of the logarithm of a drain current (I_(d)) having the highest gradient. ΔShift means the amount of change in shift value.

From the results in FIG. 64A, the amount of change in threshold voltage (ΔV_(th)) and the amount of change in shift value (ΔShift) were within ±2 V in GBT tests for the transistor included in Sample C1. Thus, it was confirmed that the transistor included in Sample C1 had high reliability.

<1-4. Repeated GBT Test>

Next, the amount of change in threshold voltage of Sample C1 when the PBTS and the NBTS were alternately performed was measured. First, I_(d)-V_(g) characteristics of the transistor were measured (initial). Then, the PBTS and the NBTS were alternately performed, and each test was performed twice in total. Each of the GBT tests was performed at a stress temperature of 60° C. at a stress time of 1 hour. The transistor measured here had a channel length L of 6 μm and a channel width W of 50 μm.

FIG. 64B shows GBT test results of Sample C1. FIG. 64B shows the threshold voltage before the stress test (initial) and the results of the GBT test in which PBTS and NBTS were repeatedly applied in the following order: PBTS, NBTS, PBTS, and NBTS.

In the case where the threshold voltage alternately increases and decreases when the PBTS and the NBTS are alternately performed, it is assumed that the threshold voltage changes because of trap and detrap of carriers by a trap level at the time of application of voltage to a gate electrode. In the case where the threshold voltage gradually increases or decreases, for example, it is assumed that the threshold voltage changes because carriers trapped by a trap level serve as fixed charges.

The results of FIG. 64B show that the transistor included in Sample C1 exhibits small variation in threshold voltage.

<1-5. Cross-Sectional Observation of Transistor>

A cross section of Sample C1 was observed. Note that the cross-sectional observation was performed with the use of a scanning transmission electron microscope (STEM). FIG. 65 shows a cross-sectional STEM image of Sample C1.

The result of the STEM image shown in FIG. 65 shows that Sample C1 manufactured in this example has a favorable cross-sectional shape.

From the above-described results, the transistor included in Sample C1 having a stacked-layer oxide semiconductor film provided between insulating films containing excess oxygen, which is one embodiment of the present invention, showed excellent electrical characteristics; that is, the improvement in reliability and the reduction in variation of electrical characteristics were achieved even when the process temperature is lowered.

Note that the structure described above in this example can be combined with any of the other embodiments and examples as appropriate.

Example 2

In this example, a transistor corresponding to the transistor 150 shown in FIGS. 2A to 2C was manufactured and the drain current-gate voltage characteristics (I_(d)-V_(g) characteristics) of the transistor were evaluated. In addition, the potential distribution of the case of a short channel length and a high drain voltage was calculated to consider the results in the I_(d)-V_(g) characteristics.

First, samples formed in this example are described.

Samples D1 and D2 described below were formed in this example. Note that Sample D1 is a transistor (Single gate structure) for comparison, and Sample D2 is a transistor (S-channel structure) of one embodiment of the present invention.

Samples D1 and D2 each include a transistor having a channel length L of 2 μm and a channel width W of 50 μm, a transistor having a channel length L of 3 μm and a channel width W of 50 μm, and a transistor having a channel length L of 6 μm and a channel width W of 50 μm.

Next, a formation method of the samples formed in this example is described below. Note that the reference numerals used for the transistor 150 in FIGS. 2A to 2C are used in the following description.

<2-1. Formation Method of Sample D1 and Sample D2>

First, the conductive film 104 was formed over the substrate 102. As the substrate 102, a glass substrate was used. As the conductive film 104, a 100-nm-thick tungsten film was formed with a sputtering apparatus.

Next, the insulating films 106 and 107 were formed over the substrate 102 and the conductive film 104. As the insulating film 106, a 400-nm-thick silicon nitride film was formed with a PECVD apparatus. As the insulating film 107, a 50-nm-thick silicon oxynitride film was formed with a PECVD apparatus.

The insulating film 106 was deposited as follows. First, a 50-nm-thick silicon nitride film was deposited under the conditions where the substrate temperature was 350° C.; a silane gas at a flow rate of 200 sccm, a nitrogen gas at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccm were introduced into a chamber; the pressure was set to 100 Pa; and an RF power of 2000 W was supplied between parallel-plate electrodes placed in a PECVD apparatus. Then, the flow rate of an ammonia gas was changed to 2000 sccm to deposit a 300-nm-thick silicon nitride film. Finally, the flow rate of an ammonia gas was changed to 100 sccm to deposit a 50-nm-thick silicon nitride film.

The insulating film 107 was deposited under the conditions where the substrate temperature was 350° C., a silane gas at a flow rate of 20 sccm and a dinitrogen monoxide gas at a flow rate of 3000 sccm were introduced into a chamber, the pressure was 40 Pa, and an RF power of 100 W was supplied between parallel-plate electrodes provided in a PECVD apparatus.

Then, the oxide semiconductor film 108 was formed over the insulating film 107. As the oxide semiconductor film 108, the oxide semiconductor film 108 b and the oxide semiconductor film 108 c were successively formed in a vacuum with a sputtering apparatus.

As the oxide semiconductor film 108 b, a 10-nm-thick IGZO film was formed under the conditions where the substrate temperature was 170° C., an argon gas at a flow rate of 140 sccm and an oxygen gas at a flow rate of 60 sccm were introduced into a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W was applied to a polycrystalline metal oxide sputtering target (having an atomic ratio of In:Ga:Zn=4:2:4.1).

As the oxide semiconductor film 108 c, a 15-nm-thick IGZO film was formed under the conditions where the substrate temperature was 170° C., an argon gas at a flow rate of 100 sccm and an oxygen gas at a flow rate of 100 sccm were introduced into a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W was applied to a polycrystalline metal oxide sputtering target (having an atomic ratio of In:Ga:Zn=1:1:1.2).

Next, the conductive films 112 a and 112 b were formed over the insulating film 107 and the oxide semiconductor film 108. As the conductive films 112 a and 112 b, a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a 100-nm-thick titanium film were successively formed in vacuum with a sputtering apparatus.

Next, a surface of the oxide semiconductor film 108 (on the back-channel side) was cleaned. As the cleaning method, a phosphoric acid solution obtained by diluting phosphoric acid (concentration of 85 vol %) 100 times with water was applied to the oxide semiconductor film 108 and the conductive films 112 a and 112 b with a spin cleaning apparatus for 15 seconds.

After that, the insulating film 114 and the insulating film 116 were formed over the insulating film 107, the oxide semiconductor film 108, and the conductive films 112 a and 112 b. As the insulating film 114, a 50-nm-thick silicon oxynitride film was formed with a PECVD apparatus. As the insulating film 116, a 400-nm-thick silicon oxynitride film was formed with a PECVD apparatus. Note that the insulating film 114 and the insulating film 116 were formed successively in vacuum with a PECVD apparatus.

The insulating film 114 was deposited under the conditions where the substrate temperature was 220° C., a silane gas at a flow rate of 50 sccm and a dinitrogen monoxide gas at a flow rate of 2000 sccm were introduced into a chamber, the pressure was 20 Pa, and an RF power of 100 W was supplied between parallel-plate electrodes provided in a PECVD apparatus. The insulating film 116 was deposited under the conditions where the substrate temperature was 220° C., a silane gas at a flow rate of 160 sccm and a dinitrogen monoxide gas at a flow rate of 4000 sccm were introduced into a chamber, the pressure was 200 Pa, and an RF power of 1500 W was supplied between parallel-plate electrodes provided in a PECVD apparatus.

Then, first heat treatment was performed at 350° C. in a nitrogen gas atmosphere for 1 hour.

Next, the opening portion 152 a reaching the conductive film 112 b and the opening portions 152 b and 152 c reaching the conductive film 104 were formed. The opening portions 152 a, 152 b, and 152 c were formed with a dry etching apparatus.

Next, an oxide semiconductor film was formed over the insulating film 116 to cover the opening portions 152 a, 152 b, and 152 c, and processed to form the oxide semiconductor films 120 a and 120 b. As the oxide semiconductor films 120 a and 120 b, a 100-nm-thick IGZO film was formed under the conditions where the substrate temperature was 170° C., an oxygen gas at a flow rate of 100 sccm was introduced into a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W was applied to a polycrystalline metal oxide sputtering target (having an atomic ratio of In:Ga:Zn=4:2:4.1).

Note that Sample D1 for comparison had a structure without the oxide semiconductor films 120 a and 120 b (i.e., a Single Gate structure).

Then, the insulating film 118 was formed over the insulating film 116 and the oxide semiconductor films 120 a and 120 b. As the insulating film 118, a 100-nm-thick silicon nitride film was formed with a PECVD apparatus. The insulating film 118 was deposited under the conditions where the substrate temperature was 350° C., a silane gas at a flow rate of 50 sccm, a nitrogen gas at a flow rate of 5000 sccm, and an ammonia gas at a flow rate of 100 sccm were introduced into a chamber, the pressure was 100 Pa, and high-frequency power of 1000 W was supplied between parallel-plate electrodes provided in a PECVD apparatus with the use of a 27.12 MHz high-frequency power source.

Then, second heat treatment was performed at 250° C. under a nitrogen atmosphere for 1 hour.

Through the above process, Sample D1 and Sample D2 of this example were formed. Note that the highest temperature through the fabrication of Sample D1 and Sample D2 was 350° C.

<2-2. I_(d)-V_(g) Characteristics>

Next, the I_(d)-V_(g) characteristics of Sample D1 and Sample D2 were measured. Note that the measurement of the I_(d)-V_(g) characteristics was performed in a manner similar to that in Example 1. The maximum field-effect mobility (μFE_max) of Samples D1 and D2 and the threshold voltages (V_(th)) of Samples D1 and D2 were obtained from the I_(d)-V_(g) characteristics and plotted in FIGS. 84A and 84B. The maximum field-effect mobility is plotted in FIG. 84A, and the threshold voltages are plotted in FIG. 84B.

As shown in FIGS. 84A and 84B, in the case of the S-channel structure, the field-effect mobility is twice or more that in the case of the Single Gate structure. Moreover, the threshold voltage does not depend on the L length of the transistor, the variation in threshold voltage is small, and the negative shift of the threshold voltage is suppressed.

<2-3. Potential Distribution Evaluation by Calculation>

Next, the effect of changing an FET structure on the potential distribution of the case of a short channel length and a high drain voltage was determined by calculation. The potential distribution of the case of a high drain voltage in each of a Single gate structure and an S-channel structure was evaluated.

Note that the calculation conditions are as follows: a bottom-gate OS-FET (IGZO composition, In:Ga:Zn=1:1:1 [atomic ratio]) structure; a channel length of 2 μm; a drain voltage (V_(d)) of 20 V; and a gate voltage (V_(g)) of −1 V. Furthermore, a simulator “ATLAS” produced by Silvaco, Inc. was used for the calculation.

FIG. 85A shows the calculation results of the potential distribution in the Single Gate structure, and FIG. 85B shows the calculation results of the potential distribution in the S-channel structure.

As shown in FIGS. 85A and 85B, the potential distribution in the OS in the S-channel structure is narrower than that in the Single Gate structure. This suggests that the DIBL effect is suppressed in the S-channel structure. Therefore, in the FET with the S-channel structure, the negative shift when a short channel length and a high drain voltage are used can be suppressed.

Note that the structure described above in this example can be combined with any of the other embodiments or examples as appropriate.

Example 3

In this example, the reason for the difference in reliability between the transistor with the Single Gate structure and the transistor with the S-channel structure in the GBT tests in Example 1 was considered.

<3-1. Consideration on PBTS>

First, PBTS was considered.

The transistor 400 shown in FIGS. 74A to 74C and the transistor 400A shown in FIGS. 75A to 75C in Embodiment 1 were considered. FIG. 86 is a conceptual diagram of a potential applied to the transistor 400, and FIG. 87 is a conceptual diagram of a potential applied to the transistor 400A.

As shown in FIG. 86, in the case where the upper gate electrode does not cover the side end portion of the active layer, positive charges are trapped over a passivation film. As shown in FIG. 87, in the case where the upper gate electrode covers the side end portion of the active layer, the electric field can be controlled by the upper gate electrode, so that trapping of positive charges can be suppressed.

<3-2. Consideration on NBTS>

Next, NBTS was considered.

FIG. 88A is a conceptual diagram of NGBT stress in a dark environment of a Single Gate structure, and FIG. 88B is a conceptual diagram of NGBT stress in a dark environment of an S-channel structure.

When the Single gate structure is used, the transistor is affected by charges that can be produced on a surface of the passivation film as shown in FIG. 88A. In contrast, when the S-channel structure is used, the upper gate electrode can shield charges that can be produced on a surface of the passivation film as shown in FIG. 88B.

Thus, the S-channel structure of the transistor of one embodiment of the present invention can increase a resistance to NBTS.

Note that the structure described above in this example can be combined with any of the other embodiments or examples as appropriate.

Example 4

In this example, samples (Samples E1 to E5) including transistors with different structures were manufactured, and the threshold voltages of the samples were compared to each other. Furthermore, I_(d)-V_(g) characteristics of transistors (Samples E4 and E5) with different structures were calculated.

<4-1. Samples E1 to E5>

First, Samples E1 to E5 are described below.

[Sample E1]

Sample E1 is a sample in which a transistor with a Single gate structure and using CAAC-IGZO was formed (hereinafter also referred to as “CAAC-IGZO, Single gate structure”).

[Sample E2]

Sample E2 is a sample in which a transistor with an S-channel structure and using CAAC-IGZO was formed (hereinafter also referred to as “CAAC-IGZO, S-channel structure”).

[Sample E3]

Sample E3 is a sample in which a transistor with a Single gate structure and using nc-IGZO was formed (hereinafter also referred to as “nc-IGZO, Single gate structure”).

[Sample E4]

Sample E4 is a sample in which a transistor with a Single gate structure and using p-channel low-temperature polycrystalline silicon was formed (hereinafter also referred to as “p-ch-LTPS, Single gate structure”).

[Sample E5]

Sample E5 is a sample in which a transistor with an S-channel structure and using p-channel low-temperature polycrystalline silicon was formed (hereinafter also referred to as “p-ch-LTPS, S-channel structure”).

<4-2. Probability Distribution of Threshold Voltage>

FIG. 89 shows the probability distribution of threshold voltages (V_(th)) of Samples E1 to E3. FIG. 90 shows the probability distribution of threshold voltages (V_(th)) of Samples E4 and E5.

The transistors of Samples E1 to E3 each had a size of L/W=6/50 μm. The drain voltage (V_(d)) in measuring the threshold voltage was 20 V. Furthermore, 70 transistors were evaluated in each of Samples E1 to E3.

As shown in FIG. 89, variation in threshold voltage is reduced more in Sample E1 (CAAC-IGZO, Single gate structure) than in Sample E3 (nc-IGZO, Single gate structure). Moreover, using Sample E2 (CAAC-IGZO, S-channel structure) instead of Sample E1 (CAAC-IGZO, Single gate structure) can further reduce the variation in threshold voltage.

As shown in FIG. 90, the variation in threshold voltage of Sample E4 (p-ch-LTPS, Single gate structure) is not greatly different from that of Sample E5 (p-ch-LTPS, S-channel structure).

<4-3. Calculation Results of I_(d)-V_(g) Characteristics>

Next, the I_(d)-V_(g) characteristics of Samples E4 and E5 were calculated. Note that in the calculation, an attention was focused on a crystal grain boundary in p-channel low-temperature polycrystalline silicon.

FIG. 91 shows the calculation results of the I_(d)-V_(g) characteristics of Samples E4 and E5.

As shown in FIG. 91, there is no large difference between the Single Gate structure and the S-channel structure in the change in the I_(d)-V_(g) characteristics depending on the position of the crystal grain boundary. Therefore, it is suggested that the variation due to the crystal grain boundary is not reduced in the case where the S-channel structure is employed in the transistor including p-channel low-temperature polycrystalline silicon. Meanwhile, it is suggested that the CAAC-IGZO is advantageous in the variation in threshold voltage because the effect of the crystal grain boundary does not exist or is extremely small.

Note that the structure described above in this example can be combined with any of the other embodiments or examples as appropriate.

This application is based on Japanese Patent Application serial no. 2015-040981 filed with Japan Patent Office on Mar. 3, 2015, Japanese Patent Application serial no. 2015-052903 filed with Japan Patent Office on Mar. 17, 2015, Japanese Patent Application serial no. 2015-127835 filed with Japan Patent Office on Jun. 25, 2015, and Japanese Patent Application serial no. 2015-239875 filed with Japan Patent Office on Dec. 9, 2015, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a transistor including: a first gate electrode; a first insulating film over the first gate electrode; an oxide semiconductor film over the first insulating film; a source electrode electrically connected to the oxide semiconductor film; a drain electrode electrically connected to the oxide semiconductor film; a second insulating film over the oxide semiconductor film; a second gate electrode over the second insulating film; and a third insulating film over the second gate electrode, wherein the oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side, and a second oxide semiconductor film over the first oxide semiconductor film, wherein the first oxide semiconductor film and the second oxide semiconductor film include In, M, and Zn, wherein M is Al, Ga, Y, or Sn, wherein, in a region of the second oxide semiconductor film, the number of atoms of the In is smaller than that in the first oxide semiconductor film, and wherein the second gate electrode includes at least one metal element included in the oxide semiconductor film.
 2. The semiconductor device according to claim 1, wherein the first oxide semiconductor film includes a region satisfying In>M, wherein the second oxide semiconductor film includes a region satisfying In≦M, and wherein M is Al, Ga, Y, or Sn.
 3. The semiconductor device according to claim 1, wherein the second gate electrode includes a third oxide semiconductor film and a fourth oxide semiconductor film over the third oxide semiconductor film, wherein the third oxide semiconductor film includes a region satisfying In≦M, wherein the fourth oxide semiconductor film includes a region satisfying In≧M, and wherein M is Al, Ga, Y, or Sn.
 4. The semiconductor device according to claim 1, wherein the oxide semiconductor film includes a crystal part having c-axis alignment.
 5. The semiconductor device according to claim 1, wherein the third insulating film includes at least one of hydrogen and nitrogen.
 6. A display device comprising: the semiconductor device according to claim 1; and a display element.
 7. A display module comprising: the display device according to claim 6; and a touch sensor.
 8. An electronic device comprising: the semiconductor device according to claim 1; and at least one of an operation key and a battery.
 9. A semiconductor device comprising: a transistor including: a first gate electrode; a first insulating film over the first gate electrode; an oxide semiconductor film over the first insulating film; a source electrode electrically connected to the oxide semiconductor film; a drain electrode electrically connected to the oxide semiconductor film; a second insulating film over the oxide semiconductor film; a second gate electrode over the second insulating film; and a third insulating film over the second gate electrode, wherein the second gate electrode is electrically connected to the first gate electrode through an opening portion in the first insulating film and the second insulating film, wherein the oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side, and a second oxide semiconductor film over the first oxide semiconductor film, wherein the first oxide semiconductor film and the second oxide semiconductor film include In, M, and Zn, wherein M is Al, Ga, Y, or Sn, wherein, in a region of the second oxide semiconductor film, the number of atoms of the In is smaller than that in the first oxide semiconductor film, and wherein the second gate electrode includes at least one metal element included in the oxide semiconductor film.
 10. The semiconductor device according to claim 9, wherein the first oxide semiconductor film includes a region satisfying In>M, wherein the second oxide semiconductor film includes a region satisfying In≦M, and wherein M is Al, Ga, Y, or Sn.
 11. The semiconductor device according to claim 9, wherein the second gate electrode includes a third oxide semiconductor film and a fourth oxide semiconductor film over the third oxide semiconductor film, wherein the third oxide semiconductor film includes a region satisfying In≦M, wherein the fourth oxide semiconductor film includes a region satisfying In≧M, and wherein M is Al, Ga, Y, or Sn.
 12. The semiconductor device according to claim 9, wherein the oxide semiconductor film includes a crystal part having c-axis alignment.
 13. The semiconductor device according to claim 9, wherein the third insulating film includes at least one of hydrogen and nitrogen.
 14. A display device comprising: the semiconductor device according to claim 9; and a display element.
 15. A display module comprising: the display device according to claim 14; and a touch sensor.
 16. An electronic device comprising: the semiconductor device according to claim 9; and at least one of an operation key and a battery
 17. A method for manufacturing a semiconductor device, comprising the steps of: forming a first gate electrode; forming a first insulating film over the first gate electrode; forming a first oxide semiconductor film over the first insulating film; forming a second oxide semiconductor film over the first oxide semiconductor film; forming a source electrode and a drain electrode over the second oxide semiconductor film; forming a second insulating film over the second oxide semiconductor film, the source electrode, and the drain electrode; forming a third oxide semiconductor film functioning as a second gate electrode, over the second insulating film; and forming a third insulating film including hydrogen, over the third oxide semiconductor film, wherein, in the step of forming the first oxide semiconductor film, the first oxide semiconductor film is formed in an atmosphere including a first oxygen gas, and the first oxygen gas is added to the first insulating film, wherein, in the step of forming the second oxide semiconductor film, the second oxide semiconductor film is formed in an atmosphere including a second oxygen gas, wherein, in the step of forming the third oxide semiconductor film, the third oxide semiconductor film is formed in an atmosphere including a third oxygen gas, and the third oxygen gas is added to the second insulating film, and wherein, in the step of forming the third insulating film or after the step of forming the third insulating film, the hydrogen is added from the third insulating film to the third oxide semiconductor film.
 18. The method for manufacturing a semiconductor device, according to claim 17, wherein the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film are each formed by a sputtering method.
 19. A method for manufacturing a semiconductor device, comprising the steps of: forming a first gate electrode; forming a first insulating film over the first gate electrode; forming a first oxide semiconductor film over the first insulating film; forming a second oxide semiconductor film over the first oxide semiconductor film; forming a source electrode and a drain electrode over the second oxide semiconductor film; forming a second insulating film over the second oxide semiconductor film, the source electrode, and the drain electrode; forming a third oxide semiconductor film functioning as a second gate electrode, over the second insulating film; forming a fourth oxide semiconductor film functioning as the second gate electrode, over the third oxide semiconductor film; and forming a third insulating film including hydrogen, over the fourth oxide semiconductor film, wherein, in the step of forming the first oxide semiconductor film, the first oxide semiconductor film is formed in an atmosphere including a first oxygen gas, and the first oxygen gas is added to the first insulating film, wherein, in the step of forming the second oxide semiconductor film, the second oxide semiconductor film is formed in an atmosphere including a second oxygen gas, wherein, in the step of forming the third oxide semiconductor film, the third oxide semiconductor film is formed in an atmosphere including a third oxygen gas, and the third oxygen gas is added to the second insulating film, wherein, in the step of forming the fourth oxide semiconductor film, the fourth oxide semiconductor film is formed in an atmosphere including a fourth oxygen gas, and wherein, in the step of forming the third insulating film or after the step of forming the third insulating film, the hydrogen is added from the third insulating film to the fourth oxide semiconductor film.
 20. The method for manufacturing a semiconductor device, according to claim 19, wherein the first oxide semiconductor film, the second oxide semiconductor film, the third oxide semiconductor film, and the fourth oxide semiconductor film are each formed by a sputtering method.
 21. The method for manufacturing a semiconductor device, according to claim 19, wherein a flow rate of the third oxygen gas is higher than a flow rate of the fourth oxygen gas. 